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TWENTIETH   CENTURY  TEXT-BOOKS 

EDITED    BY 

A.   F.   NIGHTINGALE,   Ph.  D. 

SUPERINTENDENT   OF    HIGH    SCHOOLS,    CHICAGO 


TWENTIETH    CENTURY  TEXT- BOOKS 


PLANT  RELATIONS 

A  FIRST  BOOK  OF  BOTANY 


BY 

JOHN  M.  COULTER,  A.M.,  Ph.D. 

HEAD    PROFESSOR    OF    BOTANY 
UNIVERSITY  OF  CHICAGO 


SECOND  EDITION  REVISED 


NEW    YORK 

D.    APPLETON    AND    COMPANY 

iqoi 


Copyright,  1899, 
By  D.  APPLETON  AND  COMPANY. 


PREFACE. 

The  methods  of  teaching  botany  in  secondary  schools 
are  very  diverse,  and  in  so  far  as  they  express  the  experience 
of  successful  teachers,  they  are  worthy  of  careful  considera- 
tion. As  the  overwhelming  factor  in  successful  teaching 
is  the  teacher,  methods  are  of  secondary  importance,  and 
may  well  vary.  It  is  the  purpose  of  the  present  work  to 
contribute  another  suggestion  as  to  the  method  of  teach- 
ing botany  in  secondary  schools.  The  author  does  not 
intend  to  criticise  other  methods  of  teaching,  for  each 
teacher  has  his  own  best  method,  but  it  may  be  well  to 
state  the  principles  which  underlie  the  preparation  of  this 
work. 

The  botany  is  divided  into  two  parts,  each  representing 
work  for  half  a  year.  The  two  books  are  independent, 
and  opinions  may  differ  as  to  which  should  precede.  The 
first  book,  herewith  presented,  is  dominated  by  Ecology, 
and  also  contains  certain  fundamentals  of  Physiology  that 
are  naturally  suggested.  The  second  book  will  be  domi- 
nated by  Morphology,  but  plant  structure,  function,  and 
classification  will  be  developed  together  in  an  attempt  to 
trace  the  evolution  of  the  plant  kingdom.  In  the  judg- 
ment of  the  author  Ecology  should  precede  Morphology, 
but  this  order  brings  to  Ecology  no  knowledge  of  plant 
structures  and  plant  groups,  which  is  of  course  unfortu- 
nate. The  advantages  which  seem  to  overbalance  this  dis- 
advantage are  as  follows  : 

1.  The  study  of  the  most  evident  life-relations  of 
plants  gives  a  proper  conception  of  the  place  of  plants  in 
]* 


^ 


J.O 


-685 


VI  PREFACE. 

nature,  a  fitting  background  for  subsequent  more  detailed 
studies. 

2.  Such  a  view  of  the  plant  kingdom  is  certainly  of  the 
most  permanent  value  to  those  avIio  can  give  but  a  half 
year  to  botany,  for  the  large  problems  of  Ecology  are  con- 
stantly presented  in  subsequent  experience,  when  details 
of  structure  would  be  forgotten. 

3.  The  work  in  Ecology  herein  suggested  demands  lit- 
tle or  no  use  of  the  compound  microscope,  an  instrument 
ill  adapted  to  first  contacts  with  nature. 

The  second  book  will  demand  the  use  of  the  compound 
microscope,  and  those  schools  which  possess  such  an  equip- 
ment may  prefer  to  use  that  part  first  or  exclusively. 

In  reference  to  the  use  of  this  part  something  should 
be  said,  although  such  cautions  are  reiterated  in  almost 
every  recent  publication.  A  separate  pamphlet  containing 
"  Suggestions  to  Teachers  "  who  use  this  book  has  been 
prepared,  but  a  few  general  statements  may  be  made  here. 
This  book  is  intended  to  present  a  connected,  readable 
account  of  some  of  the  fundamental  facts  of  botany,  and 
may  serve  to  give  a  certain  amount  of  information.  If  it 
performs  no  other  service  in  the  schools,  however,  its  pur- 
pose will  be  defeated.  It  is  entirely  too  compact  for  any 
such  use,  for  great  subjects,  which  should  involve  a  large 
amount  of  observation,  are  often  merely  suggested.  It  is 
intended  to  serve  as  a  supplement  to  three  far  more  im- 
portant factors  :  (1)  the  teacher,  who  must  amplify  and 
suggest  at  every  point  ;  (2)  the  laboratory,  which  must 
bring  the  pupil  face  to  face  with  plants  and  their  struc- 
tures; (3)  field-work,  which  must  relate  the  facts  observed 
in  the  laboratory  to  their  actual  place  in  nature,  and  must 
bring  new  facts  to  notice  which  can  be  observed  nowhere 
else.  Taking  the  results  obtained  from  these  three  fac- 
tors, the  book  seeks  to  organize  them,  and  to  suggest 
explanations.  It  seeks  to  do  this  in  two  ways  :  (1)  by 
means  of  the  text,  which  is  intended  to  be  clear  and  un- 


PREFACE.  VII 

technical,  but  compact ;  (2)  by  means  of  tlie  illustrations, 
which  must  be  studied  as  carefully  as  the  text,  as  they  are 
only  second  in  importance  to  the  actual  material.  Espe- 
cially is  this  true  in  reference  to  the  landscapes,  many  of 
which  cannot  be  made  a  part  of  experience. 

Thanks  are  due  to  various  members  of  the  botanical 
staff  of  the  University,  who  have  been  of  great  service  in 
offering  suggestions  and  in  preparing  illustrations.  In 
this  first  book  I  would  especially  acknowledge  the  aid  of 
Professor  Charles  E.  Barnes  and  Dr.  Henry  C.  Cowles. 

The  professional  botanist  who  may  critically  examine 
this  first  book  knows  that  Ecology  is  still  a  mass  of  incho- 
ate facts,  concerning  which  we  may  be  said  to  be  making 
preliminary  guesses.  It  seems  to  be  true,  nevertheless, 
that  these  facts  represent  the  things  best  adapted  for  pres- 
entation in  elementary  work.  The  author  has  been  com- 
pelled to  depend  upon  the  writings  of  Warming  and  of 
Kerner  for  this  fundamental  material.  From  the  work  of 
the  latter,  and  from  the  recent  splendid  volume  of  Schim- 
per,  most  useful  illustrations  have  been  obtained.  The 
number  of  original  illustrations  is  large,  but  those  obtained 
elsewhere  are  properly  credited.  John  M.  Coulter. 

The  University  of  Chicago,  May,  1S99. 


PREFACE   TO   THE    SECOND   EDITION. 

In"  this  edition  the  first  eleven  chapters  remain  practi- 
cally as  they  were,  with  the  exception  of  such  corrections 
and  additions  as  could  be  made  upon  the  plates,  and  a  few 
changes  of  illustrations.  The  remaining  chapters,  however, 
dealing  with  plant  societies,  are  essentially  recast  both  in 
text  and  illustrations.  Especially  is  this  true  of  the  meso- 
phyte  and  halophyte  societies.  This  has  been  made  neces- 
sary by  the  recent  rapid  development  of  the  subject,  by  a 
larger  field  experience,  and  by  the  availability  of  more  suit- 
able illustrations.  J.  M.  C. 
The  University  ok  Chicago,  May,  1901. 


CONTENTS. 

CHAPTER  PAGE 

I. — Introduction 1 

II. — Foliage  leaves  :     The  light-relation           ...  6 

III. — Foliage  leaves  :     Function,  structure,  and  protection  28 

IY. — Shoots 53 

V.-Roots 89 

VI. — Reproductive  organs 109 

VII. — Flowers  and  insects 123 

VIII. — An  individual  plant  in  all  of  its  relations      .         .  138 

IX. — The  struggle  for  existence           .....  142 

X. — The  nutrition  of  plants 149 

XI. — Plant  societies  :     Ecological  factors           .        .         .  1G2 

XII. — Hydrophyte  societies c        .170 

XIII. — Xerophyte  societies 193 

XIV. — Mesopiiyte  societies         .                 233 

Index 239 


BOTANY 

.-  PLANT    RELATIONS 


CHAPTER    I. 

INTRODUCTION. 

1.  General  relations. — Plants  form  the  natural  covering 
of  the  earth's  surface.  So  generally  is  this  true  that  a  land 
surface  without  plants  seems  remarkable.  Not  only  do 
plants  cover  the  land,  but  they  abound  in  waters  as  well, 
both  fresh  and  salt  waters.  They  are  wonderfully  varied  in 
size,  ranging  from  huge  trees  to  forms  so  minute  that  the 
microscope  must  be  used  to  discover  them.  They  are  also 
exceedingly  variable  in  form,  as  may  be  seen  by  comparing 
trees,  lilies,  ferns,  mosses,  mushrooms,  lichens,  and  the 
green  thready  growths  ((dgce)  found  in  water. 

2.  Plant  societies. — One  of  the  most  noticeable  facts  in 
reference  to  plants  is  that  they  do  not  form  a  monotonous 
covering  for  the  earth's  surface,  but  that  there  are  forests  in 
one  place,  thickets  in  another,  meadows  iu  another,  swamp 
growths  iu  another,  etc.  In  this  way  the  general  appear- 
ance of  vegetation  is  exceedingly  varied,  and  each  appear- 
ance tells  of  certain  conditions  of  living.  These  groups  of 
plants  living  together  in  similar  conditions,  as  trees  and 
other  plants  in  a  forest,  or  grasses  and  other  plants  in  a 
meadow,  are  known  implant  societies.     These  societies  are  as 


PROPERTY  LIBRARY 
H.  C  Stat*  Collegt 


2  PLANT   RELATIONS. 

numerous  as  are  the  conditions  of  living,  and  it  may  be  said 
that  each  society  has  its  own  special  regulations,  which  ad- 
mit certain  plants  and  exclude  others.  The  study  of  plant 
societies,  to  determine  their  conditions  of  living,  is  one  of 
the  chief  purposes  of  botanical  field  work. 

3.  Plants  as  living  things. — Before  engaging  in  a  study 
of  societies,  however,  one  must  discover  in  a  general  way 
how  the  individual  plant  lives,  for  the  plant  covering  of  the 
earth's  surface  is  a  living  one,  and  plants  must  always  be 
thought  of  as  living  and  at  work.  They  are  as  much  alive 
as  are  animals,  and  so  far  as  mere  living  is  concerned  they 
live  in  much  the  same  way.  Xor  must  it  be  supposed  that 
animals  move  and  plants  do  not,  for  while  more  animals  than 
plants  have  the  power  of  moving  from  place  to  place,  some 
plants  have  this  power,  and  those  that  do  not  can  move  cer- 
tain parts.  The  more  we  know  of  living  things  the  more  is 
it  evident  that  life  processes  are  alike  in  them  all,  whether 
plants  or  animals.  In  fact,  there  are  some  living  things 
about  which  we  are  uncertain  whether  to  regard  them  as 
plants  or  animals. 

4.  The  plant  body. — Every  plant  has  a  body,  which  may 
be  alike  throughout  or  may  be  made  up  of  a  number  of 
different  parts.  When  the  green  thready  plants  (algm),  so 
common  in  fresh  water,  are  examined,  the  body  looks  like 
a  simple  thread,  without  any  special  parts  ;  but  the  body  of 
a  lily  is  made  up  of  such  dissimilar  parts  as  root,  stem, 
leaf,  and  flower  (see  Figs.  75,  144,  155,  174).  The  plant 
without  these  special  parts  is  said  to  be  simple*  the  plant 
with  them  is  called  comjrfex^  The  simple  plant  lives  in 
the  same  way  and  does  the  same  kind  of  work,  so  far  as 
living  is  concerned,  as  does  the  complex  plant.  The  differ- 
ence is  that  in  the  case  of  the  simple  plant  its  whole  body 
does  every  kind  of  work  ;  while  in  the  complex  plant 
different  kinds  of  work  are  done  by  different  regions  of  the 
body,  and  these  regions  come  to  look  unlike  when  differ- 
ent shapes  are  better  suited  to  different  work,  as  in  the 


INTRODUCTION.  3 

case  of  a  leaf  and  a  root,  two  regions  of  the  body  doing 
different  kinds  of  work. 

5.  Plant  organs. — These  regions  of  the  plant  body  thns 
set  apart  for  special  purposes  are  called  organ*.      The  sim- 
plest of  plants,   therefore,   do   not  have  distinct   organs, 
while  the  complex  plants  may  have  several  kinds  of  organs. 
All  plants  are  not  either  very  simple  or  very  complex,  but 
beginning  with  the  simplest  plants  one  may  pass  to  others 
not  quite  so  simple,  then  to  others  more  complex,  and  so 
on  gradually  until  the  most  complex  forms  are  reached. 
This  process  of  becoming  more  and  more  complex  is  known 
as  differentiation^  which  simply  means  the  setting  apart  of 
different  regions  of  the  body  to  do  different  kinds  of  work. 
The  advantage  of  this  to  the  plant  becomes  plain  by  using 
the  common  illustration  of  the  difference  between  a  tribe 
of  savages  and  a  civilized  community.      The  savages  all  do 
the  same  things,  and  each  savage  does  everything.     In  the 
civilized  community   some   of   the  members  are  farmers, 
others  bakers,  others  tailors,  others  butchers,  etc.     This  is 
what  is  known  as  "  division  of  labor,"  and  one  great  advan- 
tage it  has  is  that  every  kind  of  work  is  better  done.     Dif- 
ferentiation of  organs  in  a  plant  means  to  the  plant  just 
what  division  of  labor  means  to  the  community  ;  it  results 
in  more  work,  and  better  work,  and  new  kinds  of  work. 
The  very  simple  plant  resembles  the  savage  tribe,  the  com- 
plex plant  resembles  the  civilized  community.     It  must  be 
understood,  however,  that  in  the  case  of  plants  the  differ- 
entiation referred  to  is  one  of  organs  and  not  of  individuals. 
6.  Plant  functions. — Whether  plants  have  many  organs, 
or  few  organs,  or  no  organs,  it  should  be  remembered  that 
they  are  all  at  work,  and  are  all  doing  the  same  essential 
things.     Although  many  different  kinds  of  work  are  being 
carried  on  by  plants,  they  may  all  be  put  under  two  heads, 
nutrition  and  reproduction.     Every  plant,  whether  simple 
or  complex,  must  care  for  two  things  :    (1)  its  own  support 
(nutrition),   and  (2)   the  production  of    other  plants  like 


4  PLANT   RELATIONS. 

itself  (reproduction).  To  the  great  work  of  nutrition  many 
kinds  of  work  contribute,  and  the  same  is  true  of  repro- 
duction. Nutrition  and  reproduction,  however,  are  the 
two  primary  kinds  of  work,  and  it  is  interesting  to  note 
that  the  first  advance  in  the  differentiation  of  a  simple 
plant  body  is  to  separate  the  nutritive  and  reproductive 
regions.  In  the  complex  plants  there  are  nutritive  organs 
and  reproductive  organs  ;  by  which  is  meant  that  there  are 
distinct  organs  which  specially  contribute  to  the  Avork  of 
nutrition,  and  others  which  are  specially  concerned  with 
the  work  of  reproduction.  The  different  kinds  of  work  are 
conveniently  spoken  of  as  functions^  each  organ  having  one 
or  more  functions. 

7.  Life-relations.— In  its  nutritive  and  reproductive  work 
the  plant  is  very  dependent  upon  its  surroundings.  It 
must  receive  material  from  the  outside  and  get  rid  of  waste 
material ;  and  it  must  leave  its  offspring  in  as  favorable 
conditions  for  living  as  possible.  As  a  consequence,  every 
organ  holds  a  definite  relation  to  something  outside  of  it- 
self, known  as  its  life -relation^.  For  example,  green  leaves 
are  definitely  related  to  light,  many  roots  are  related  to 
soil,  certain  plants  are  related  to  abundant  water,  some 
plants  are  related  to  other  plants  or  animals  (living  as 
parasites),  etc.  A  plant  with  several  organs,  therefore, 
may  hold  a  great  variety  of  life-relations,  and  it  is  quite  a 
complex  problem  for  such  a  plant  to  adjust  all  of  its  parts 
properly  to  their  necessary  relations.  The  study  of  the 
life-relations  of  plants  is  a  division  of  Botany  known  as 
Ecology,  and  presents  to  us  many  of  the  most  important 
problems  of  plant  life. 

It  must  not  be  supposed  that  any  plant  or  organ  holds 
a  perfectly  simple  life-relation,  for  it  is  affected  by  a  great 
variety  of  things.  A  root,  for  instance,  is  affected  by  light, 
gravity,  moisture,  soil  material,  contact,  etc.  Every  or- 
gan, therefore,  must  adjust  itself  to  a  very  complex  set  of 
life-relations,  and  a  plant  with  several  organs  has  so  many 


INTRODUCTION.  O 

delicate  adjustments  to  care  for  that  it  is  really  impossi- 
ble, as  yet,  for  us  to  explain  why  all  of  its  parts  are  placed 
just  as  they  are.  In  the  beginning  of  the  study  of  plants, 
only  some  of  the  most  prominent  functions  and  life-rela- 
tions can  be  considered.  In  order  to  do  this,  it  seems  bet- 
ter to  begin  with  single  organs,  and  afterwards  these  can 
be  put  together  in  the  construction  of  the  whole  plant. 


CHAPTER   II. 

FOLIAGE  LEAVES:    THE  LIGHT-RELATION. 

8.  Definition. — A  foliage  leaf  is  the  ordinary  green  leaf, 
and  is  a  very  important  organ  in  connection  with  the  work 
of  nutrition.  It  must  not  be  thought  that  the  work  done  by 
such  a  leaf  cannot  be  done  by  green  plants  which  have  no 
leaves,  as  the  algae,  for  example.  A  leaf  is  simply  an  or- 
gan set  apart  to  do  such  work  better.  In  studying  the 
work  of  a  leaf,  therefore,  we  have  certain  kinds  of  work 
set  apart  more  distinctly  than  if  they  were  confused  with 
other  kinds.  For  this  reason  the  leaf  is  selected  as  an  in- 
troduction to  some  of  the  important  work  carried  on  by 
plants,  but  it  must  not  be  forgotten  that  a  plant  does  not 
need  leaves  to  do  this  work  ;  they  simply  enable  it  to  work 
more  effectively. 

9.  Position. — It  is  easily  observed  that  foliage  leaves 
grow  only  upon  stems,  and  that  the  stems  which  bear  them 
always  expose  them  to  light ;  that  is,  such  leaves  are  aerial 
rather  than  subterranean  (see  Figs.  1,  75,  174).  Many 
stems  grow  underground,  and  such  stems  either  bear  no 
foliage  leaves,  or  are  so  placed  that  the  foliage  leaves  are 
sent  above  the  surface,  as  in  most  ferns  and  many  plants  of 
the  early  spring  (see  Figs.  45,  46,  144). 

10.  Color. — Another  fact  to  be  observed  is  that  foliage 
leaves  have  a  characteristic  green  color,  a  color  so  universal 
that  it  has  come  to  be  associated  with  plants,  and  espe- 
cially with  leaves.  It  is  also  evident  that  this  green  color 
holds  some  necessary  relation  to  light,  for  the  leaves  of 
plants  grown  in  the  dark,  as  potatoes  sprouting  in  a  cellar, 


FOLIAGE    LEAVES:    THE    LIGHT-RELATION.  7 

do  not  develo]o  this  color.  Even  when  leaves  have  devel- 
oped the  green  color  they  lose  it  if  deprived  of  light,  as  is 
shown  by  the  process  of  blanching  celery,  and  by  the  effect 
on  the  color  of  grass  if  a  board  has  lain  upon  it  for 
some  time.  It  seems  plain,  therefore,  that  the  green  color 
found  in  working  foliage  leaves  depends  upon  light  for  its 
existence. 

We  conclude  that  at  least  one  of  the  essential  life-rela- 
tions of  a  foliage  leaf  is  what  may  be  called  the  light-rela- 
tion. This  seems  to  explain  satisfactorily  why  such  leaves 
are  not  developed  in  a  subterranean  position,  as  are  many 
stems  and  most  roots,  and  why  plants  which  produce  them 
do  not  grow  in  the  dark,  as  in  caverns.  The  same  green, 
and  hence  the  same  light-relation,  is  observed  in  other 
parts  of  the  plant  as  well,  and  in  plants  Avithout  leaves,  the 
only  difference  being  that  leaves  display  it  most  conspicu- 
ously. Another  indication  that  the  green  color  is  con- 
nected with  light  may  be  obtained  from  the  fact  that  it  is 
found  only  in  the  surface  region  of  plants.  If  one  cuts 
across  a  living  twig  or  into  a  cactus  body,  the  green  color 
will  be  seen  only  in  the  outer  part  of  the  section.  The  con- 
clusion is  that  the  leaf  is  a  special  organ  for  the  light-re- 
lation. Plants  sometimes  grow  in  such  situations  that  it 
would  be  unsafe  for  them  to  display  leaves,  or  at  least  large 
leaves.  In  such  a  case  the  work  of  the  leaves  can  be  thrown 
upon  the  stem.  A  notable  illustration  of  this  is  the  cactus 
plant,  which  produces  no  foliage  leaves,  but  whose  stem  dis- 
plays the  leaf  color. 

11.  An  expanded  organ. — Another  general  fact  in  refer- 
ence to  the  foliage  leaf  is  that  in  most  cases  it  is  an  expanded 
organ.  This  means  that  it  has  a  great  amount  of  surface 
exposed  in  comparison  witli  its  mass.  As  this  form  is  of 
such  common  occurrence  it  is  safe  to  conclude  that  it  is  in 
some  way  related  to  the  work  of  the  leaf,  and  that  whatever 
work  the  leaf  does  demands  an  exposure  of  surface  rather 
than  thickness  of  body.     It  is  but  another  step  to  say  that 


8 


PLANT   KELATIONS. 


the  amount  of  work  an  active  leaf  can  do  will  depend  in 
part  upon  the  amount  of  surface  it  exposes. 


THE    LIGHT-RELATION. 


12.  The  general  relation. — The  ordinary  position  of  the 
foliage  leaf  is  more  or  less  horizontal.  This  enables  it  to 
receive  the  direct  rays  of  light  upon  its  upper  surface.     In 

this  way  more  rays  of 
light  strike  the  leaf  sur- 
face than  if  it  stood  ob- 
liquely or  on  edge.  It  is 
often  said  that  leaf  blades 
are  so  directed  that  the 
flat  surface  is  at  right 
angles  to  the  incident 
rays  of  light.  While  this 
may  be  true  of  horizon- 
tal leaves  in  a  general 
way,  the  observation  of 
almost  any  plant  will 
show  that  it  is  a  very 
general  statement,  to 
which  there  are  numerous 
exceptions  (see  Fig.  1). 
Leaves  must  be  arranged 
to  receive  as  much  light 
as  possible  to  help  in 
their  work,  but  too  much 
light  will  destroy  the 
green  substance  (cliloro- 
phyll),  which  is  essential 
to  the  work.  The  adjust- 
ment to  light,  therefore, 
is  a  delicate  one,  for 
there  must  be  just  enough 


Fig.  1.  The  leaves  of  this  plant  (Ficus)  are 
in  general  horizontal,  but  it  will  be  seen 
that  the  lower  ones  are  directed  down- 
ward, and  that  the  leaves  become  more 
horizontal  as  the  stem  is  ascended.  It 
will  also  be  seen  that  the  leaves  are  so 
broad  that  there  are  few  vertical  rows. 


FOLIAGE    LKAVES:    THE    LIGHT-RELATION. 


9 


and  not  too  much.  The  danger  from  too  much  light  is 
not  the  same  in  the  case  of  all  leaves,  even  on  the  same 
plant,  for  some  are  more  shaded  than  others.  Leaves  also 
have  a  way  of  protecting  themselves  from  too  intense  light 
by  their  structure,  rather  than  by  a  change  in  their  posi- 
tion. It  is  evident,  therefore,  that  the  exact  position  which 
any  particular  leaf  holds  in  relation  to  light  depends  upon 
many  circumstances,  and  cannot  be  covered  by  a  general 
rule,  except  that  it  seeks  to  get  all  the  light  it  can  without 
danger. 

13.  Fixed  position. — Leaves  differ  very  much  in  the  power 
of  adjusting  their  position  to  the  direction  of  the  light. 


Fig.  2.    The  day  and  night  positions  of  the  leaves  of  a  member  (Amicia)  of  the  pea 
family.— After  Strasburger. 

Most  leaves  when  fully  grown  are  in  a  fixed  position  and 
cannot  change  it,  however  unfavorable  it  may  prove  to  be, 
except  as  they  are  blown  about.  Such  leaves  are  said  to 
have  fixed  light  positions.  This  position  is  determined  by 
the  light  conditions  that  prevailed  while  the  leaf  was  grow- 
ing and  able  to  adjust  itself.  If  these  conditions  continue, 
the  resulting  fixed  position  represents  the  best  one  that  can 
be  secured  under  the  circumstances.  The  leaf  may  not 
receive  the  rays  of  light  directly  throughout  the  whole 
period  of  daylight,  but  its  fixed  position  is  such  that  it 
probably  receives  more  light  than  it  would  in  any  other 
position  that  it  could  secure. 


10 


PLANT   EELATIONS. 


14.  Motile  leaves. — There  are  leaves,  however,  which 
have  no  fixed  light  position,  hut  are  so  constructed  that 
they  can  shift  their  position  as  the  direction  of  the  light 
changes.     Such  leaves  are  not  in  the  same  position  in  the 

afternoon  as  in  the 
forenoon,  and  their 
night  position  may  he 
very  different  from 
either  (see  Figs.  %,  3a, 
ob,  4).  Some  of  the 
common  house  plants 
show  this  power.  In 
the  case  of  the  com- 
mon Oxalis  the  night 
position  of  the  leaves 
is  remarkahly  different 


Fig.  3a.    The  day  position  of  the  leaves  of  redbud 
(Ce?xis). — After  Arthur. 


from  the  position  in  light. 
If  such  a  plant  is  exposed 
to  the  light  in  a  window  and 
the  positions  of  the  leaves 
noted,  and  then  turned 
half  way  around,  so  as  to 
bring  the  other  side  to  the 
light,  the  leaves  may  be 
observed  to  adjust  them- 
selves gradually  to  the 
changed  light-relations. 

15.    Compass    plants. — A 
striking    illustration    of    a 

special  light  position  is  found  in  the  so-called  "compass 
plants."  The  best  known  of  these  plants  is  the  rosin-weed 
of  the  prairie  region.  Growing  in  situations  exposed  to 
intense  light,  the  leaves  are  turned  edgewise,  the  flat  faces 
being  turned  away  from  the  intense  rays  of  midday,  and 
directed  towards  the  rays  of  less  intensity  ;  that  is,  those  of 


Fig.  3b.    The  night  position  of  the  leaves 
of  redbud  (Cercis).— After  Arthur. 


FOLIAGE    LEAVES:    THE    LIGHT-RELATION. 


11 


Fig.  4.  Two  sensitive  plants,  showing  the  motile  leaves.  The  plant  to  the  left  has  its 
leaves  and  numerous  leaflets  expanded  ;  the  one  to  the  right  shows  the  leaflets 
folded  together  and  the  leaves  drooping. — After  Kekner. 


the  morning  and  evening  (see  Fig.  170).  As  a  result,  the 
apex  of  the  leaf  points  in  a  general  north  or  south  direction. 
It  is  a  significant  fact  that  when  the  plant  grows  in  shaded 
places  the  leaves  do  not  assume  any  such  position.  It 
seems  evident,  therefore,  that  the  position  has  something 

of  too  intense  light.     It 


to  do  with  avoiding  the  danger 


12 


PLANT  RELATIONS. 


Fig.  5.  The  common  prickly  lettuce  (Lactuca 
Scariola),  showing  the  leaves  standing  edge- 
wise, and  in  a  general  north  and  south  plane. 
—After  Arthur  and  MacDougal. 


must  not  be  supposed 
that  there  is  any  ac- 
curacy in  the  north  or 
south  direction,  as  the 
edgewise  position 
seems  to  be  the  signifi- 
cant one.  In  the  ros- 
in-weed probably  the 
north  and  south  direc- 
tion is  the  prevailing 
one ;  but  in  the  prickly 
lettuce,  a  very  common 
weed  of  waste  grounds, 
and  one  of  the  most 
striking  of  the  compass 
plants,  the  edgewise 
position  is  frequently 
assumed  without  any 
special  reference  to  the 
north  or  south  direc- 
tion of  the  apex  (see 
Fig.  5). 

10.  Heliotropism. — 
The  property  of  leaves 
and  of  other  organs 
of  responding  to  light 
is  known  as  heliotro- 
pism,  and  it  is  one 
of  the  most  important 
of  those  external  influ- 
ences to  which  plant 
organs  respond  (see 
Figs.  6,  43). 

It  should  be  under- 
stood clearly  that  this 
is  but  a  slight  glimpse 


FOLIAGE    LEAVES:    THE    LIGHT-RELATION. 


13 


Fig.  6.    These  plants  are  growing  near  a  window.    It  will  be  noticed  that  the  stems 
bend  strongly  towards  the  light,  and  that  the  leaves  face  the  light. 

of  the  most  obvious  relations  of  foliage  leaves  to  light,  and 
that  the  important  part  which  heliotropism  plays,  not  only 
in  connection  with  foliage  leaves,  hut  also  in  connection 
with  other  plant  organs,  is  one  of  the  most  important  and 
extensive  subjects  of  plant  physiology. 


RELATION    OF    LEAVES   TO    ONE    ANOTHER. 

A.    Oil  erect  stems. 

In  view  of  what  has  been  said,  it  would  seem  that  the 
position  of  foliage  leaves  on  the  stem,  and  their  relation  to 
one  another,  must  be  determined  to  some  extent  by  the 
necessity  of  a  favorable  light-relation.  It  is  apparent  that 
the  conditions  of  the  problem  are  not  the  same  for  an  erect 
as  for  a  horizontal  stem. 

17.  Relation  of  breadth  to  number  of  vertical  rows.— 
Upon  an  erect  stem  it  is  observed  that  the  leaves  are  usu- 


14 


PLANT   RELATIONS. 


ally  arranged  in  a  definite  number  of  vortical  rows.  It  is 
to  the  advantage  of  the  plant  for  these  leaves  to  shade  one 
another  as  little  as  possible.  Therefore,  the  narrower  the 
leaves,  the  more  numerous  may  be  the  vertical  rows  (see 

Figs.  7,  8)  ;  and 
the  broader  the 
leaves  the  fewer 
the  vertical  rows 
(see  Fig.  1).  A 
relation  exists, 
therefore,  .be- 
tween the  breadth 
of  leaves  and  the 
number  of  verti- 
cal rows,  and  the 
meaning  of  this 
becomes  plain 
when  the  light-re- 
lation is  consid- 
ered. 

18.  Relation  of 
length  to  the  dis- 
tance between 
leaves  of  the  same 
row. — The  leaves 
in  a  vertical  row 
may  be  close  together  or  far  apart.  If  they  should  be  close 
together  and  at  the  same  time  long,  it  is  evident  that  they 
will  shade  each  other  considerably,  as  the  light  cannot  well 
strike  in  between  them  and  reach  the  surface  of  the  lower 
leaf.  Therefore,  the  closer  together  the  leaves  of  a  verti- 
cal row,  the  shorter  are  the  leaves  ;  and  the  farther  apart 
the  leaves  of  a  row,  the  longer  may  they  be.  Short  leaves 
permit  the  light  to  strike  between  them  even  if  they  are 
close  together  on  the  stem  ;  and  long  leaves  permit  the 
same  thing  only  when  they  are  far  apart  on  the  stem.     A 


Fig.  7. 


An  Easter  lily,   showing  narrow  leaves  and 
numerous  vertical  rows. 


FOLIAGE    LEAVES  :    THE    LIGHT-RELATION. 


15 


relation  is  to  be  observed,  therefore,  between  the  length 
of  leaves  and  their  distance  apart  in  the  same  vertical  row. 
The  same  kind  of  relation  can  be  observed  in  reference 
to  the  breadth  of  leaves,  for  if  leaves  are  not  only  short  but 
narrow  they  can  stand  very  close  together.  It  is  thus  seen 
that  the  length  and  breadth  of  leaves,  the  number  of  ver- 
tical rows  on  the  stem,  and  the  distance  between  the  leaves 


Fig.  8.    A  dragon-tree,  showing  narrow  leaves  extending  in  all  directions,  and  numer- 
ous vertical  rows. 


of  any  row,  all  have  to  do  with  the  light-relation  and  are 
answers  to  the  problem  of  shading. 

1!).  Elongation  of  the  lower  petioles. — There  is  still 
another  common  arrangement  by  which  an  effective  light- 
relation  is  secured  by  leaves  which  are  broad  and  placed 
close  together  on  the  stem.  In  such  a  ease  the  stalks 
{petioles)  of  the  lower  leaves  become  longer  than  those 
above  and  thus  thrust  their  blades  beyond  the  shadow  (see 
Fig.   9).      It  may  be  noticed  that  it  is  very  common    to 


16 


PLANT   RELATIONS. 


find  the  lowest  leaves  of  a  plant  the  largest  and  with  the 
longest  petioles,  even  when  the  leaves  are  not  very  close 
together  on  the  stem. 

It  must  not  be  supposed  that  by  any  of  these  devices 
shading  is  absolutely  avoided.  This  is  often  impossible  and 
sometimes   undesirable.     It  simply    means  that   by  these 


mag-'*, 


Fig.  9.    A  plant  (Saintpaulia)  with  the  lower  petioles  elongated,  thrusting  the  blades 
beyond  the  shadow  of  the  upper  leaves.    A  loose  rosette. 


arrangements  the  most  favorable  light-relation  is  sought  by 
avoiding  too  great  shading. 

20.  Direction  of  leaves. — Not  only  is  the  position  on  the 
stem  to  be  observed,  but  the  direction  of  leaves  may  result 
in  a  favorable  relation  to  light.  It  is  it  very  common  thing 
to  find  a  plant  with  a  cluster  of  comparatively  large  leaves 
at  or  near  the  base,  where  they  are  in  no  danger  of  shading 
other  leaves,  and  with  the  stem  leaves  gradually  becoming 


FOLIAGE   LEAVES  :    THE    LIGHT-RELATION. 


17 


smaller  and  less  horizontal  toward  the  apex  of  the  stem 
(see  Figs.  10,  13).  The  common  shepherd's  purse  and  the 
mullein  may  be  taken  as  illustrations.  By  this  arrange- 
ment all  the  leaves  are  very 
completely  exposed  to  the 
light. 

21.  The  rosette  habit.— 
The  habit  of  producing  a 
cluster  or  rosette  of  leaves 
at  the  base  of  the  stem  is 
called  the  rosette  habit. 
Often  this  rosette  of  leaves 
at  the  base,  frequently  lying 
flat  on  the  ground  or  on  the 
rocks,  includes  the  only  fo- 
liage leaves  the  plant  pro- 
duces. It  is  evident  that  a 
rosette,  in  which  the  leaves 
must  overlap  one  another 
more  or  less,  is  not  a  very 
favorable  light  arrange- 
ment, and  therefore  it  must 
be  that  something  is  being 
provided  for  besides  the 
light-relation  (see  Figs.  11, 
12,  13).  What  this  is  will 
appear   later,    but   even   in 

this  comparatively  unfavorable  light  arrangement,  there  is 
evident  adjustment  to  secure  the  most  light  possible  under 
the  circumstances.  The  lowest  leaves  of  the  rosette  are 
the  longest,  and  the  upper  (or  inner)  ones  become  gradu- 
ally shorter,  so  that  all  the  leaves  have  at  least  a  part 
of  the  surface  exposed  to  light.  The  overlapped  base  of 
such  leaves  is  not  expanded  as  much  as  the  exposed  apex, 
and  hence  they  are  mostly  narrowed  at  the  base  and  broad 
at  the  apex.     This  narrowing  at   the   base    is   sometimes 


Fig.  10.  A  plant  (Echereria)  with  fleshy 
leaves,  showing  large  horizontal  ones 
at  base,  and  others  becoming  smaller 
and  more  directed  upward  as  the 
stem  is  ascended. 


IS 


PLANT   RELATIONS. 


carried  so  far  that  most  of  the  part  which  is  covered  is 
but  a  stem  (petiole)  for  the  upper  part  (blade)  which  is 
exposed. 

In  many  plants  which  do  not  form  close  rosettes  a  gen- 


Fig.  11.  A  group  of  live-for-evers,  illustrating  the  rosette  habit  and  the  light-relation. 
In  the  rosettes  it  will  be  observed  how  the  leaves  are  fitted  together  and  diminish 
in  size  inwards,  so  that  excessive  shading  is  avoided.  The  individual  leaves  also 
become  narrower  where  they  overlap,  and  are  broadest  where  they  are  exposed  to 
light.    In  the  background  is  a  plant  showing  leaves  in  very  definite  vertical  rows. 


eral  rosette  arrangement  of  the  leaves  may  be  observed  by 
looking  down  upon  them  from  above  (see  Fig.  0),  as  in  some 
of  the  early  buttercups  which  are  so  low  that  the  large 
leaves  would  seriously  shade  one  another,  except  that  the 
lower  leaves  have  longer  petioles  than  the  upper,  and  so 
reach  beyond  the  shadow. 


FOLIAGE    LEAVES:    THE    LIGHT-RELATION. 


19 


Fig.  12.  Two  clumps  of  rosettes  of  the  house  leek  (Sempervirutn),  the  one  to  the 
right  showing  the  compact  winter  condition,  the  one  to  the  left  with  rosettes  more 
open  after  being  kept  indoors  for  several  days. 


22.  Branched  leaves. — Another  notable  feature  of  foliage 
leaves,  which  has  something  to  do  with  the  light-relation, 
is  that  on  some  plants  the  blade  does  not  consist  of  one 
j)iece,  but  is  lobed  or  even  broken  up  into  separate  pieces. 
When  the  divisions  are  distinct  they  are  called  leaflets,  and 
every  gradation  in  leaves  can  be  found,  from  distinct  leaf- 
lets to  lobed  leaves,  toothed  leaves,  and  finally  those  whose 
margins  are  not  indented  at  all  [entire).  This  difference 
in  leaves  probably  has 
more  important  rea- 
sons than  the  light- 
relation,  but  its  sig- 
nificance may  be  ob- 
served in  this  connec- 
tion. In  those  plants 
whose  leaves  are  un- 
divided, the  leaves 
generally  either  di- 
minish in  size  toward 
the  top    of  the  stem, 

or  the  lower  ones  de-     v     1Q    , 

Jig.  13.    The  leaves  <>t  a  bellflower  (Campanula), 

Velop  longer   petioles.  showing  the  rosette  arrangement.    The  lower 

In  this  case  the  gen-  ^u^  m  ^^  '"J"?  CarTng  ""'ir 

°  blades  beyond  the  shadow  of  the  blades  above, 

eral    outline    of     the  —After  Kernek. 


Fig.  14.  A  group  of  leaves,  showing  how- 
dangerous  shading.  It  will  be  seen  th 
are  towards  the  bottom  of  the  group. 


branched  leaves  overtop  each  other  without 
it  the  larger  blades  or  less-branched  leaves 


FOLIAGE    LEAVES:    THE    LIGHT-RELATION. 


21 


plant  is  conical,  a  form  very  common  in  herbs  with  entire 
or  nearly  entire  leaves.  In  plants  whose  leaf  blades  are 
broken  up  into  leaflets  (compound  or  branched  leaves), 
however,  no  such  diminution  in  size  toward  the  top  of  the 
stem  is  necessary  (see  Fig.  17),  though  it   may  frequently 


plant  showing  much-branched  leaves,  which  occur  in  great  profusion  with- 
out cutting  off  the  light  from  one  another. 


occur.  When  a  broad  blade  is  broken  up  into  leaflets 
the  danger  of  shading  is  very  much  less,  as  the  light  can 
strike  through  between  the  upper  Leaflets  and  reach  the 
leaflets  below.  On  the  lower  leaves  there  will  be  splotches 
of  light  and  shadow,  but  they  will  shift  throughout  the 
day,  so  that  probably  a  large  part  of  the  leaf  will  receive 
light  at  some  time    during  the  day   (see  Fig.    14).     The 


22 


PLAXT   RELATIONS. 


general  outline  of  such  a  plant,  therefore,  is  usually  not 
conical,  as  in  the  other  case,  but  cylindrical  (see  Figs.  4, 
15,  16,  22,  45,  83,  96,  155,  162,  174  tor  branched  leaves). 

Many  other  factors  enter  into  the  light-relation  of  foli- 
age leaves  upon  erect  stems,  but  those  given  may  suggest 


Fig.  16.    A  cycad,  showing  much-branched  leaves  and  palm-like  habit. 

observation  in  this  direction,  and  serve  to  show  that  the 
arrangement  of  leaves  in  reference  to  light  depends  upon 
many  things,  and  is  by  no  means  a  fixed  and  indifferent 
thing.  The  study  of  any  growing  plant  in  reference  to  this 
one  relation  presents  a  multitude  of  problems  to  those  who 
know  how  to  observe. 


B.    On  horizontal  stems. 

23.  Examples  of  horizontal  stems,  that  is,  stems  exposed 
on  one  side  to  the  direct  light,  will  be  found  in  the  case  of 
many  branches  of  trees,  stems  prostrate  on  the  ground,  and 


FOLLAGE    LEAVES      THE    LIGHT-RELATION. 


23 


stems  against  a  support,  as  the  ivies.  It  is  only  necessary 
to  notice  how  the  leaves  are  adjusted  to  light  on  an  erect 
stem,  and  then  to  bend  the 
stem  into  a  horizontal  posi- 
tion or  against  a  support,  to 
realize  how  unfavorable  the 
same  arrangement  would 
be,  and  how  many  new  ad- 
justments must  be  made. 
The  leaf  blades  must  all  be 
brought  to  the  light  side  of 
the  stem,  so  far  as  possible, 
and  those  that  belong  to 
the  lower  side  of  the  stem 
must  be  fitted  into  the 
spaces  left  by  the  leaves 
which  belong  to  the  upper 
side.  This  may  be  brought 
about  by  the  twisting  of 
the  stem,  the  twisting  of 
the  petioles,  the  bending  of 
the  blade  on  the  petiole, 
the  lengthening  of  petioles, 
or  in  some  other  way. 
Every  horizontal  stem  has 
its  own  special  problems  of 
leaf  adjustment  Avhich  may 
be  observed  (see  Figs.  18, 
50). 

Sometimes  there  is  not 
space  enough  for  the  full 
development  of  every  blade, 
and  smaller  ones  are  fitted 
into  the  spaces  left  by  the  larger  ones  (see  Fig.  21).  This 
sometimes  results  in  what  are  called  unequally  paired  leaves, 
where  opposite  leaves  develop  one  large  blade  and  one  small 


Fig.  17.  A  chrysanthemum,  showing 
lobed  leaves,  the  rising  of  the  petioles 
to  adjust  the  blades  to  light,  and  the 
general  cylindrical  habit. 


24 


PLANT   KELATIONS. 


one.  Perhaps  the  most  complete  fitting  together  of  leaves 
is  found  in  certain  ivies,  where  a  regular  layer  of  angular 
interlocking  leaves  is  formed,  the  leaves  fitting  together  like 


Fig.  18.    A  plant  (Pellionia)  with  drooping  stems,  showing  how  the  leaves  are  all 
brought  to  the  lighted  side  and  fitted  together. 

the  pieces  of  a  mosaic.  In  fact  such  an  arrangement  is 
known  us  the  mosaic  arrangement,  and  involves  such  an 
amount   of  twisting,  displacement,  elongation   of  petioles, 


26 


PLANT   RELATIONS. 


Fig.  20.  A  spray  of  maple,  showing  the  adjustment  of  the  leaves  in  size  and  position 
of  blades  and  length  of  petioles  to  secure  exposure  to  light  on  a  horizontal  stem.— 
After  Kerner. 


etc.,  as  to  give  ample  evidence  of  the  effort  put  forth  by 
plants  to  secure  a  favorable  light-relation  for  their  foliage 


Fig.  21.  Two  plants  showing  adjustment  ot  leaves  on  a  horizontal  stem.  The  plant 
to  the  left  is  nightshade,  in  which  small  blades  are  fitted  into  spaces  left  by  the 
large  ones.  The  plant  to  the  right  is  Selaginella,  in  which  small  leaves  are  dis- 
tributed along  the  sides  of  the  stem,  and  others  are  displayed  along  the  upper  sur- 
face.—After  Kerner. 


FOLIAGE    LEAVES:    THE   LIGHT-RELATION. 


27 


leaves  (see  Figs.  19,  22).  In  the  case  of  ordinary  shade  trees 
every  direction  of  branch  may  be  found,  and  the  resulting 
adjustment  of  leaves  noted  (see  Fig.  20). 

Looking  up  into  a  tree  in  full  foliage,  it  will  be  noticed 
that  the  horizontal   branches  are    comparatively  bare  be- 


Fig.  22.    A  mosaic  of  fern  (Adiantum)  leaflets. 


neath,  while  the  leaf  blades  have  been  carried  to  the  upper 
side  and  have  assumed  a  mosaic  arrangement. 

Sprays  of  maidenhair  fern  (see  Fig.  22)  show  a  remark- 
able amount  of  adjustment  of  the  leaflets  to  the  light  side. 
Another  group  of  fern-plants,  known  as  club-mosses,  has 
horizontal  stems  clothed  with  numerous  very  small  leaves. 
These  leaves  may  be  seen  taking  advantage  of  all  the  space 
on  the  lighted  side  (see  Fig.  21). 


CHAPTER  III. 

FOLIAGE    LEAVES:    FUNCTION,    STRUCTURE,    AND    PROTEC- 
TION. 

A.     Functions  of  foliage  leaves. 

24.  Functions  in  general. — AVe  have  observed  that  foliage 
leaves  are  light-related  organs,  and  that  this  relation  is  an 
important  one  is  evident  from  the  various  kinds  of  adjust- 
ment used  to  secure  it.  We  infer,  therefore,  that  for  some 
important  function  of  these  leaves  light  is  necessary.  It 
would  be  hasty  to  suppose  that  light  is  necessary  for  every 
kind  of  work  done  by  a  foliage  leaf,  for  some  forms  of  work 
might  be  carried  on  by  the  leaf' that  light  neither  helps  nor 
hinders.  Foliage  leaves  are  not  confined  to  one  function, 
but  are  concerned  in  a  variety  of  processes,  all  of  which 
have  to  do  with  the  great  work  of  nutrition.  Among  the 
variety  of  functions  which  belong  to  foliage  leaves  some  of 
the  most  important  may  be  selected  for  mention.  It  will 
be  possible  to  do  little  more  than  indicate  these  functions 
until  the  plant  with  all  its  organs  is  considered,  but  some 
evidence  can  be  obtained  that  various  processes  are  taking 
place  in  the  foliage  leaf. 

25.  Photosynthesis. — The  most  important  function  of  the 
foliage  leaf  may  be  detected  by  a  simple  experiment.  If 
an  actively  growing  water  plant  be  submerged  in  water  in  a 
glass  vessel,  and  exposed  to  the  light,  bubbles  may  be  seen 
coming  from  the  leaf  surfaces  and  rising  through  the  water 
(see  Fig.  23).  The  water  is  merely  a  device  by  which  the 
bubbles  of  gas  may  be  seen.     If  the  plant  is  very  active  the 


FOLIAGE    LEAVES  :     FUNCTION,    STRUCTURE.    ETC.         29 

bubbles  are  numerous.  That  this  activity  holds  a  definite 
relation  to  light  may  be  proved  by  gradually  removing  the 
vessel  containing  the  plant  from  the  light.  As  the  light 
diminishes  the  bubbles  diminish  in  number,  and  when  a 


Fig.  23.    An  experiment  to  illustrate  the  giving  ( 

synthesis. 


of  oxygen  in  tho  process  of  photo- 


certain  amount  of  darkness  has  been  reached  the  bubbles 
will  cease  entirely.  If  now  the  vessel  be  brought  back 
gradually  into  the  light,  the  bubbles  will  reappear,  more 
and  more  numerous  as  the  light  increases.  That  this  gas 
being  given  off  is  oxygen  may  be  proved  by  collecting  the 


30  PLANT   RELATIONS. 

bubbles  in  a  test  tube,  as  in  an  ordinary  chemical  experi- 
ment for  collecting  gas  over  water,  and  testing  it  in  the 
usual  way. 

Some  very  important  things  are  learned  by  this  experi- 
ment. It  is  evident  that  some  process  is  going  on  within 
the  leaves  which  needs  light  and  which  results  in  giving  olf 
oxygen.  It  is  further  evident  that  as  oxygen  is  eliminated, 
the  process  indicated  is  dealing  with  substances  which 
contain  more  oxygen  than  is  needed.  The  amount  of 
oxygen  given  off  may  be  taken  as  the  measure  of  the  work. 
The  more  oxygen,  the  more  work ;  and,  as  we  have  observed, 
the  more  light,  the  more  oxygen;  and  no  light,  no  oxygen. 
Therefore,  light  must  be  essential  to  the  work  of  which  the 
elimination  of  oxygen  is  an  external  indication.  That  this 
process,  whatever  it  may  be,  is  so  essentially  related  to 
light,  suggests  the  idea  that  it  is  the  special  process  which 
demands  that  the  leaf  shall  be  a  light-related  organ.  If  so, 
it  is  a  dominating  kind  of  work,  as  it  chiefly  determines 
the  life-relations  of  foliage  leaves. 

The  process  thus  indicated  is  known  as  photosynthesis, 
and  the  name  suggests  that  it  has  to  do  with  the  arrange- 
ment of  material  with  the  help  of  light.  It  is  really  a  pro- 
cess of  food  manufacture,  by  Avhich  raw  materials  are  made 
into  plant  food.  This  process  is  an  exceedingly  important 
one,  for  upon  it  depend  the  lives  of  all  plants  and  animals. 
The  foliage  leaves  may  be  considered,  therefore,  as  special 
organs  of  photosynthesis.  They  are  special  organs,  not  ex- 
clusive organs,  for  any  green  tissue,  whether  on  stem  or  fruit 
or  any  part  of  the  plant  body,  may  do  the  same  work.  It 
is  at  once  apparent,  also,  that  during  the  night  the  process 
of  photosynthesis  is  not  going  on,  and  therefore  during  the 
night  oxygen  is  not  being  given  off. 

Another  part  of  this  process  is  not  so  easily  observed,  but 
is  so  closely  related  to  the  elimination  of  oxygen  that  it 
must  be  mentioned.  Carbon  dioxide  occurs  in  the  air  to 
which  the  foliage  leaves  are  exposed.     It  is  given  off  from 


FOLIAGE    LEAVES:    FUNCTION,    STRUCTURE,    ETC.        31 

our  lungs  in  breathing,  and  also  comes  off  from  burning 
wood  or  coal.  It  is  a  common  waste  product,  being  a  com- 
bination of  carbon  and  oxygen  so  intimate  that  the  two 
elements  are  separated  from  one  another  with  great  dif- 
ficulty. During  the  process  of  photosynthesis  it  has  been 
discovered  that  carbon  dioxide  is  being  absorbed  from  the 
air  by  the  leaves.  As  this  gas  is  absorbed  chiefly  by  green 
parts  and  in  the  light,  in  just  the  conditions  in  which  oxy- 
gen is  being  given  off,  it  is  natural  to  connect  the  two,  and 
to  infer  that  the  process  of  photosynthesis  involves  not  only 
the  green  color  and  the  light,  but  also  the  absorption  of 
carbon  dioxide  and  the  elimination  of  oxygen. 

When  we  observe  that  carbon  dioxide  is  a  combination 
of  carbon  and  oxygen,  it  seems  reasonable  to  suppose  that 
the  carbon  and  oxygen  are  separated  from  one  another  in 
the  plant,  and  that  the  carbon  is  retained  and  the  oxygen 
given  back  to  the  air.  The  process  of  photosynthesis  may 
be  partially  defined,  therefore,  as  the  breaking  up  of  carbon 
dioxide  by  the  green  parts  of  the  plants  in  the  presence  of 
light,  the  retention  of  the  carbon,  and  the  elimination  of 
the  oxygen.  The  carbon  retained  is  combined  into  real 
plant  food,  in  a  way  to  be  described  later.  We  may  con- 
sider photosynthesis  as  the  most  important  function  of  the 
foliage  leaf,  of  which  the  absorption  of  carbon  dioxide  and 
the  evolution  of  oxygen  are  external  indications  ;  and  that 
light  and  chlorojmyll  are  in  some  way  essentially  connected 
with  it. 

2G.  Transpiration. — One  of  the  easiest  things  to  observe 
in  connection  with  a  working  leaf  is  the  fact  that  it  gives 
off  moisture.  A  simple  experiment  may  demonstrate  this. 
If  a  glass  vessel  (bell  jar)  be  inverted  over  a  small  active 
plant  the  moisture  is  seen  to  condense  on  the  glass,  and 
even  to  trickle  down  the  sides.  A  still  more  convenient  way 
to  demonstrate  this  is  to  select  a  single  vigorous  leaf  with 
a  good  petiole  ;  pass  the  petiole  through  a  perforated  card- 
board resting  upon  a  tumbler  containing  water,  and  invert 


32  PLANT    RELATIONS. 

a  second  tumbler  over  the  blade  of  the  leaf,  which  projects 
above  the  cardboard  (see  Fig.  24).  It  will  be  observed  that 
moisture  given  off  from  the  surface  of  the  working  leaf  is 
condensed  on  the  inner  surface  of  the  inverted  tumbler. 
The  rani  board  is  to  shut  off  evaporation  from  the  water 
in  the  lower  tumbler. 

When  tin'  amount  of  water  given  off  by  a  single  leaf  is 
noted,  some  vague  idea  may  be  formed  as  to  the  amount  of 
moisture  given  off  by  a  great  mass  of  vegetation,  such  as  a 
meadow  or  a  forest.  It  is  evident  that  green  plants  at 
work  are  contributing  a  very  large  amount  of  moisture  to 
the  air  in  the  form  of  water  vapor,  moisture  which  has 
been  absorbed  by  some  region  of  the  plant.  The  foli- 
age leaf,  therefore,  may  be  regarded  as  an  organ  of 
transpiration,  not  that  the  leaves  alone  are  engaged  in 
transpiration,  for  many  parts  of  the  plant  do  the  same 
thing,  but  because  the  foliage  leaves  are  the  chief  seat  of 
transpiration. 

In  case  the  leaves  are  submerged,  as  is  true  of  many 
plants,  it  is  evident  that  transpiration  is  practically  checked, 
for  the  leaves  are  already  bathed  with  water,  and  under  such 
circumstances  water  vapor  is  not  given  off.  It  is  evident 
that  under  such  circumstances  leaf  work  must  be  carried 
on  without  transpiration.  In  some  cases,  as  in  certain 
grasses,  fuchsias,  etc.,  drops  of  water  are  extruded  at  the 
apex  of  the  leaf,  or  at  the  tips  of  the  teeth.  This  process 
is  called  (juttafion,  and  by  means  of  it  a  good  deal  of 
water  passes  from  the  leaf.  It  is  specially  used  by  shade 
plants,  which  live  in  conditions  which  do  not  favor  tran- 
spiration. 

27.  Respiration. — Another  kind  of  work  also  may  be  de- 
tected in  the  foliage  leaf,  but  not  so  easily  described.  In 
fact  it  escaped  the  attention  of  botanists  long  after  they 
had  discovered  photosynthesis  and  transpiration.  It  is  work 
that  goes  on  so  long  as  the  leaf  is  alive,  never  ceasing  day 
or  night.     The  external  indication  of  it  is  the  absorption 


Fig.  24.     Experiment  illustrating  transpiration. 


34  PLANT   RELATIONS. 

of  oxygen  and  the  giving  out  of  carbon  dioxide.  It  will  be 
noted  at  once  that  this  is  exactly  the  reverse  of  what  takes 
place  in  photosynthesis.  During  the  day,  therefore,  carbon 
dioxide  and  oxygen  are  both  being  absorbed  and  evolved. 
It  will  also  be  noted  that  the  taking  in  of  oxygen  and  the 
giving  out  of  carbon  dioxide  is  just  the  sort  of  exchange 
which  takes  place  in  our  own  respiration.  In  fact  this  pro- 
cess is  also  called  respiration  in  plants.  It  does  not  depend 
upon  light,  for  it  goes  on  in  the  dark.  It  does  not  depend 
upon  chlorophyll,  for  it  goes  on  in  plants  and  parts  of  plants 
which  are  not  green.  It  is  not  peculiar  to  leaves,  but  goes 
on  in  every  living  part  of  the  plant.  A  process  which  goes 
on  without  interruption  in  all  living  plants  and  animals 
must  be  very  closely  related  to  their  living.  We  conclude, 
therefore,  that  while  photosynthesis  is  peculiar  to  green 
plants,  and  only  takes  place  in  them  when  light  is  present, 
respiration  is  necessary  to  all  plants  in  all  conditions,  and 
that  when  it  ceases  life  must  soon  cease.  The  fact  is, 
respiration  supplies  the  energy  which  enables  the  living 
substance  to  work. 

Once  it  was  thought  that  plants  differ  from  animals 
in  the  fact  that  plants  absorb  carbon  dioxide  and  give  off 
oxygen,  while  animals  absorb  oxygen  and  give  off  carbon 
dioxide.  It  is  seen  now  that  there  is  no  such  difference, 
but  that  respiration  (absorption  of  oxygen  and  evolution  of 
carbon  dioxide)  is  common  to  both  plants  and  animals. 
The  difference  is  that  green  plants  have  the  added  work  of 
photosynthesis. 

We  must  also  think  of  the  foliage  leaf,  therefore,  as  a 
respiring  organ,  because  very  much  of  such  work  is  done 
by  it,  but  it  must  be  remembered  that  respiration  is  going 
on  in  every  living  part  of  the  plant. 

This  by  no  means  completes  the  list  of  functions  that 
might  be  made  out  for  foliage  leaves,  but  it  serves  to  indi- 
cate both  their  peculiar  work  (photosynthesis)  and  the  fact 
that  they  are  doing  other  kinds  of  work  as  well. 


FOLIAGE    LEAVES:    FUNCTION,    STRUCTURE,    ETC.        35 

B.   Structure  of  foliage  leaves. 

28.   Gross  structure. — It  is  evident  that  the  essential  part 
of  a  foliage  leaf  is  its  expanded  portion  or  Made.    Often  the 

leaf  is  all  blade  (see  Figs.  7, 
8, 18)  ;  frequently  there  is  a 
longer  or  shorter  leaf-stalk 
{petiole)  which  helps  to  put 


Fig.  25.  Two  types  of  leaf  venation.  The  figure  to  the  left  is  a  leaf  of  Solomon's 
seal  (Polygonatum),  and  shows  the  principal  veins  parallel,  the  very  minute  cross 
veinlets  being  invisible  to  the  naked  eye,  being  a  monocotyl  type.  The  figure  to 
the  right,  is  a  leaf  of  a  willow,  and  6hows  netted  veins,  the  main  central  vein  (mid- 
rib) sending  out  a  series  of  parallel  branches,  which  are  connected  with  one  another 
by  a  network  of  veinlets,  being  a  dicotyl  type.— After  Ettingshausen. 

the  blade  into  better  light-relation  (see  Figs.  1,  9,  17,  20, 
2G);  and  sometimes  there  are  little  leaf -like  appendages  (stip- 
ules)  on  the  petiole  where  it  joins  the  stem,  whose  func- 
tion is  not  always  clear.  Upon  examining  the  blade  it 
is  seen  to  consist  of  a  green  substance    through  which  a 


36 


PLANT   RELATIONS. 


fiamework  of  veins  is  variously  arranged.  The  large  veins 
which  enter  the  blade  send  off  smaller  branches,  and  these 
send  off  still  smaller  ones,  until  the  smallest  veinlets  are 

invisible,  and  the 
framework  is  a 
close  network  of 
branching  veins. 
This  is  plainly 
shown  by  a  "skel- 
eton "  leaf,  one 
which  has  been  so 
treated  that  all 
the  green  sub- 
stance has  disap- 
peared, and  only 

veins  remains.  It 
will  be  noticed 
that  in  some 
leaves  the  veins 
and  veinlets  are 
very  prominent, 
in  others  only 
the  main  veins 
are  prominent, 
while  in  some  it 
is  hard  to  detect 
any  veins  (see 
Figs.  25,  20). 

29.  Significance 
of  leaf  veins. — It 
is  clear  that  the 
framework  of  veins  is  doing  at  least  two  things  for  the 
blade:  (1)  it  mechanically  supports  the  spread  out  green  sub- 
stance ;  and  (2)  it  conducts  material  to  and  from  the  green 
substance.       So  complete  is  the  network  of  veins  that  this 


Fig.  26.  A  leaf  of  hawthorn,  showing  a  short  petiole,  and 
a  hroad  toothed  blade  with  a  conspicuous  network  of 
veins.  Note  the  relation  between  the  veins  and  the 
teeth.— After  Strasburger. 


FOLIAGE    LEAVES:    FUNCTION,    STRUCTURE,    ETC.        37 

support  and  conduction  are  very  perfect  (see  Fig.  27).  It 
is  also  clear  that  the  green  substance  thus  supported  and 
supplied  with  material  is  the  important  part  of  the  leaf,  the 
part  that  demands  the  light-relation.  Study  the  various 
plans  of  the  vein  systems  in  Figs.  3,  9,  13,  18,  19,  20,  21, 
25,  2$,  51,  70,  76,  S2,  S3,  92,  161. 


Pig.  27.    A  plant  (Fittonia)  whose  leaves  show  a  network  of  veins,  and  also  an  adjust- 
ment to  one  another  to  form  a  mosaic. 

30.  Epidermis. — If  a  thick  leaf  he  taken,  such  as  that 
of  a  hyacinth,  it  will  he  found  possible  to  peel  off  from 
its  surface  a  delicate  transparent  skin  (epidermis).  This 
epidermis  completely  covers  the  leaf,  and  generally  shows 
no  green  color.  It  is  a  protective  covering,  but  at  the  same 
time  it  must  not  completely  shut  off  the  green  substance 
beneath  from  the  outside.  It  is  found,  therefore,  that 
three  important  parts  of  an  ordinary  foliage  leaf  are  :  (1) 


38 


PLANT   RELATIONS. 


Fig.  28.  Cells  of  the  epidermis 
of  Maranta,  showing  the 
interlocking  walls,  and  a 
stoma  0)  with  its  two  guard- 
cells. 


a  network  of  veins  ;  (2)  a  green  substance   (mesophyll)  in 

the  meshes  of  the  network  ;  and  (3)  over  all  an  epidermis. 
31.  Stomata. — If  a  compound  microscope  is  used,  some 

very  important  additional  facts  may  be  discovered.  The 
thin,  transparent  epidermis  is 
found  to  be  made  up  of  a  layer  of 
cells  which  fit  closely  together, 
sometimes  dovetailing  with  each 
other.  Curious  openings  in  the 
epidermis  will  also  be  discovered, 
sometimes  in  very  great  numbers. 
Guarding  each  opening  are  two 
crescent-shaped  cells,  known  as 
guard- cells,  and  between  them  a 
slit-like  opening  leads  through  the 
epidermis.  The  whole  apparatus 
is  known  as  a  stoma  (plural 
stomata),     which     really     means 

"  mouth,"  of  which  the  guard-cells  might  be  called  the 

lips  (see  Figs.  28,  29).     Sometimes  stomata  are  found  only 

on  the  under  side  of  the  leaf,  sometimes  only 

on  the  upper  side,  and  sometimes  on  both 

sides. 

The  important  fact  about  stomata  is  that 

the  guard-cells  can  change  their  shape,  and 

so  regulate  the  size  of  the  opening.    It  is  not 

certain  just  why  the  guard-cells  change  their 

shape  and  just  what  stomata  do  for  leaves. 

They   are  often  called   "  breathing  pores," 

but  the  name  is  very  inappropriate.    Stomata 

are  not  peculiar  to  the  epidermis  of  foliage 

leaves,  for  they  are  found  in  the  epidermis 

of   any  green  part,  as  stems,  young  fruit, 

etc.    It  is  evident,  therefore,  that  they  hold 

an  important  relation  to  green  tissue  which 

is  covered  by  epidermis.    Also,  if  we  examine 


Fig.  2'.).  A  single 
stoma  from  the 
epidermis  of  a 
lily  leaf,  show- 
ing the  t  w  o 
guard-cells  full 
of  chlorophyll, 
and  the  small 
slit-like  opening 
between. 


FOLIAGE   LEAVES:    FUNCTION,    STRUCTURE,    ETC.        39 

foliage  leaves  and  other  green  parts  of  plants  which  live 
submerged  in  water,  we  find  that  the  epidermis  contains 
no  stomata.  Therefore,  stomata  hold  a  definite  relation 
to  green  parts  covered  by  epidermis  only  when  this  epider- 
mis is  exposed  to  the  air. 

It  would  seem  that  the  stomata  supply  open  passage- 
ways for  material  from  the  green  tissue  through  the  epider- 
mis to  the  air,  or  from  the  air  to  the  green  tissue,  or  both. 
It  will  be  remembered,  however,  that  quite  a  number  of 
substances  are  taken  into  the  leaf  and  given  out  from  it, 
so  that  it  is  hard  to  determine  whether  the  stomata  are 
specially  for  any  one  of  these  movements.  For  instance, 
the  leaf  gives  out  moisture  in  transpiration,  oxygen  in 
photosynthesis,  and  carbon  dioxide  in  respiration  ;  while  it 
takes  in  carbon  dioxide  in  photosynthesis,  and  oxygen  in 
respiration.  It  is  thought  stomata  specially  favor  transpira- 
tion, and,  if  so,  "breathing  pores"  is  not  a  happy  phrase, 
for  they  certainly  assist  in  the  other  exchanges. 

32.  Mesophyll. — If  a  cross-section  be  made  of  an  ordi- 
nary foliage  leaf,  such  as  that  of  a  lily,  the  three  leaf 
regions  can  be  seen  in  their  proper  relation  to  each  other. 
Bounding  the  section  above  and  below  is  the  layer  of  trans- 
parent epidermal  cells,  pierced  here  and  there  by  stomata, 
marked  by  their  peculiar  guard-cells.  Between  the  epi- 
dermal layers  is  the  green  tissue,  known  as  the  mesophyll, 
made  up  of  cells  which  contain  numerous  small  green 
bodies  which  give  color  to  the  whole  leaf,  and  arc  known  as 
chlorophyll  bodies  or  chloroplasts. 

The  mesophyll  cells  are  usually  arranged  differently  in 
the  upper  and  lower  regions  of  the  leaf.  In  the  upper 
region  the  cells  are  elongated  and  stand  upright,  present- 
ing their  narrow  ends  to  the  upper  leaf  surface,  forming 
the  palisade  tissue.  In  the  lower  region  the  cells  are  irreg- 
ular, and  so  loosely  arranged  as  to  leave  passageAvays  for  air 
between,  forming  the  *}nm(jy  tissue.  The  air  spaces  among 
the  cells  communicate  with  one  another,  so  that  a  system  of 
4 


40 


PLANT   RELATIONS. 


air  chambers  extends  throughout  the  spongy  mesophyll. 
It  is  into  this  system  of  air  chambers  that  the  stomata 
open,  and  so  they  are  put  into  direct  communication  with 
the  mesophyll  or  working  cells.  The  peculiar  arrangement 
of  the  upper  mesophyll,  to  form  the  palisade  tissue,  has  to 
do  with  the  fact  that  that  surface  of  the  leaf  is  exposed  to 
the  direct  rays  of  light.  This  light,  so  necessary  to  the 
mesophyll,  is  also  dangerous  for  at  least  two  reasons.     If 


Fig.  30.  A  section  through  the  leaf  of  lily,  showing  upper  epidermis  (ue),  lower  epi- 
dermis (le)  with  its  stomata  (st),  mesophyll  (dotted  cells)  composed  of  the  palisade 
region  (p)  and  the  spongy  region  (sp)  with  airspaces  among  the  cells,  and  two 
veins  (v)  cut  across. 


the  light  is  too  intense  it  may  destroy  the  chlorophyll,  and 
the  heat  may  dry  out  the  cells.  By  presenting  only  nar- 
row ends  to  this  direct  light  the  cells  are  less  exposed  to 
intense  light  and  heat.     Study  Fig.  30. 

33.  Veins. — In  the  cross-section  of  the  leaf  there  will 
also  be  seen  here  and  there,  embedded  in  the  mesophyll, 
the  cut  ends  of  the  veinlets,  made  up  partly  of  thick- 
walled  cells,  which  hold  the  leaf  in  shape  and  conduct 
material  to  and  from  the  mesophyll  (see  Fig.  30). 


FOLIAGE   LEAVES:    FUNCTION,    STRUCTURE,    ETC.       41 


C.    Leaf  protection. 

34.  Need  of  protection. — Such  an  important  organ  as 
the  leaf,  with  its  delicate  active  cells  well  displayed,  is  ex- 
posed to  numerous  dangers.  Chief  among  these  dangers 
are  intense  light,  drought,  and  cold.  All  leaves  are  not 
exposed  to  these  dangers.  For  example,  plants  which  grow 
in  the  shade  are  not  in  danger  from  intense  light  ;  many 

water  plants  are  not  in  danger 
from  drought  ;  and  plants  of 
the  tropical  lowlands  are  in  no 


Fig.  31.  Sections  through  leaves  of  the  same  plant,  showing  the  effect  of  exposure  to 
light  upon  the  structure  of  the  mesophyll.  In  both  cases  os  indicates  upper  surface, 
and  us  under  surface.  In  the  section  at  the  left  the  growing  leaf  was  exposed  to 
direct  and  intense  sunlight,  and,  as  a  consequence,  all  of  the  mesophyll  cells  have 
assumed  the  protected  or  palisade  position.  In  the  section  at  the  right  the  leaf  was 
grown  in  the  shade,  and  none  of  the  mesophyll  cells  have  organized  in  palisade 
fashion. — After  Staul. 

danger  from  cold.  The  danger  from  all  these  sources  is  he- 
cause  of  the  large  surface  with  no  great  thickness  of  body, 
and  the  protection  against  all  of  them  is  practically  the 
same.  Most  of  the  forms  of  protection  can  be  reduced 
to  two  general  plans:  (1)  the  development  of  protective 
structures  between  the  endangered  mesophyll  and  the  air  ; 
(2)  the  diminution  of  the  exposed  surface. 

35.  Protective  structures. — The  palisade  arrangement  of 
mesophyll  may  be  regarded  as  an  adaptation  for  protection, 


42 


PLANT   RELATIONS. 


but  it  usually  occurs,  and  does  not  necessarily  imply  ex- 
treme conditions  of  any  kind.  However,  if  the  cells  of  the 
palisade  tissue    are    unusually  narrow   and   elongated,   or 


Fig.  32.     Section  through  a  portion  of  the  leaf  of  the  yew  (Taxus),  showing  cuticle 
(c),  epidermis  (e),  and  the  upper  portion  of  the  palisade  cells  (p). 

form  two  or  three  layers,  we  might  infer  the  probability  of 
exposure  to  intense  light  or  drought.  The  accompanying 
illustration  (Fig.  31)  shows  in  a  striking  way  the  effect  of 
light  intensity  upon  the  structure  of  the  mesophyll,  by 
contrasting  leaves  of  the  same  plant  exposed  to  the  extreme 
conditions  of  light  and  shade. 

The  most  usual  structural  adaptations,  however,  are 
connected  with  the  epidermis.  The  outer  walls  of  the  epi- 
dermal cells  may  become  thickened,  sometimes  excessively 

so ;  the  other  epidermal 
pfo  walls  may  also  become 
more  or  less  thickened; 
or  even  what  seems  to 
be  more  than  one  epi- 
dermal layer  is  found 
protecting  the  meso- 
phyll.  If  the  outer 
walls  of  the  epidermal 
cells  continue  to 
thicken,  the  outer  re- 
gion of  the  thick  wall 
loses  its  structure 
and  forms  the  cuticle, 
which   is   one   of    the 


tion  of  the  leaf  of 


carnation,  showing  the  heavy  cuticle  (cw) 
formed  by  the  outer  walls  of  the  epidermal 
cells  (ep).  Through  the  cuticle  a  passageway 
leads  to  the  stoma,  whose  two  guard-cells  are 
seen  lying  between  the  two  epidermal  cells 
shown  in  the  figure.  Below  the  epidermal 
cells  some  of  the  palisade  cells  (pal)  are  shown 
containing  chloroplasts,  and  below  the  stoma 
is  seen  the  air  chamber  into  which  it  opens. 


FOLIAGE    LEAVES:    FUNCTION,    STRUCTURE,    ETC.        43 


best  protective  substances  (see  Fig. 
32).  Sometimes  this  cuticle  be- 
comes so  thick  that  the  passage- 
ways through  it  leading  down  to 
the  stomata  become  regular  canals 
(see  Fig.  33). 

Another  very  common  protective 
structure  upon  leaves  is  to  be  found 
in  the  great  variety  of  hairs  de- 
veloped by  the  epidermis.  These 
may  form  but  a  slightly  downy 
covering,  or  the  leaf  may  be  cov- 
ered by  a  woolly  or  felt-like  mass 
so  that  the  epidermis  is  entirely 
concealed.  The  common  mullein 
is  a  good  illustration  of  a  felt- 
covered  leaf  (see  Fig.  30).  In  cold 
or  dry  regions  the  hairy  covering 
of  leaves  is  very  noticeable,  often 
giving  them  a  brilliant  silky  white  or  bronze  look  (see 
Figs.  34,  35).  Sometimes,  instead  of  a  hair-like  cover- 
ing, the  epidermis  develops  scales  of  various  patterns, 
often  overlapping,  and  forming  an  excellent  protection 
(see  Fig.  37).  In  all  these  cases  it  should  be  remembered 
that  these  hairs  and  scales  may  serve  other  purposes  also, 
as  well  as  that  of  protection. 

36.  Diminution 
of  exposed  surface. — 
it  will  be  impossible 
to  give  more  than  a 
few  illustrations  of 
this  large  subject. 
In  very  dry  regions 
it  has  always  been 
noticed  that  the 
leaves  are  small  and 


Fig.  34.  A  hair  from  the  leaf 
of  Potentilla.  It  is  seen 
to  grow  out  from  the  epi- 
dermis. 


Fig.  35.  A  section  through  the  leaf  of  bush  clover 
;  I.tsjh  deza),  show  ing  upper  and  lower  epidermiB, 
palisade  cells,  and  cells  of  the  spongy  region. 
The  lower  epidermis  produces  numerous  hairs 
which  bend  sharply  and  lie  along  the  leaf  surface 
(appressed),  forming  a  close  covering. 


u 


PLANT   RELATIONS. 


Fig.  3G.     A  branching  hair  from  the  leaf  of  common  mullein.     The  whole  plant  has  a 
felt-like  covering  composed  of  such  hairs. 


it 


comparatively  thick,  although  they  may  he  very  numerous 
(see  Figs.  4,  172).      In  this  way  each  leaf  exposes  a  small 

surface  to  the   dry- 

v/;\  :^\.  ^j  mg  aii"  and  intense 

-"V;  '.i**,  ? *       ..-;/'  ;  sunlight.      In    our 

..•    hU'jf^l'''-  southwestern    dry 

regions  the  cactus 
abounds,  plants 
which  have  reduced 
their  leaves  so  much 
that  they  are  no 
longer  used  for 
chlorophyll  work, 
and  are  not  usually 
recognized  as  leaves. 
In  their  stead  the 
globular  or  cylin- 
1    >"•'  drical   or   flattened 

stems  are  green  and 

Fig.  37.    A  scale  from  the  leaf  of  Shepherdia.  These 

6cales  overlap  and  form  a  complete  covering.  do    leat    WOI*K    (.TlgS. 


:  mm 


<i*lP 


c3  -3     i 


M    ~    O 

o    o 


O        C         S_ 


O     ci 

,d  g 

CD      £ 


2i 

p   o 


s  °  I  » 

5   «   §  a 


J14 

■K      S 

O     ■"    03 


Fig.  39.  A  group  of  cactus  forms  (slender  cylindrical,  columnar, 
and  globular),  all  of  them  spiny  and  without  leaves  ;  an  agave  in 
front ;  clusters  of  yucca  flowers  in  the  background. 


FOLIAGE   LEAVES:    FUNCTION,    STRUCTURE,    ETC.        47 

38,  39,  40,  190,  191,  192,  193).  In  the  same  regions  the 
agaves  and  yuccas  retain  their  leaves,  but  they  become  so 
thick  that  they  serve  as  water  reservoirs  (see  Figs.  38,  39, 


Fig.  40.     A  globular  cactus,  showing  the  ribbed  stem,  the  strong  spines,  and  the  entire 
absence  of  leaves. 

194).  In  all  these  cases  this  reduced  surface  is  supple- 
mented by  palisade  tissue,  very  thick  epidermal  walls,  and 
an  abundant  cuticle. 

37.   Rosette  arrangement. — The  rosette  arrangement   of 
leaves  is  a  very  common  method  of  protection  used  by 


48 


PLANT   KELATIONS. 


small  plants  growing  in  exposed  situations,  as  bare  rocks 
and  sandy  ground.  The  cluster  of  leaves,  flat  upon  the 
ground,  or  nearly  so,  and  more  or  less  overlapping,  is  very 
effectively  arranged  for  resisting  intense  light  or  drought 
or  cold  (see  Figs.  11,  12,  48). 

38.  Protective  positions. — In    other  cases,  a  position  is 
assumed  by  the  leaves  which  directs  their  flat 
surfaces  so   that   they  are   not  exposed  to  the 
most  intense  rays  of  light.     The  so-called 


Fig.  41.  A  leaf  of  a  sensitive  plant  in  two  conditions.  In  the  figure  to  the  left  the 
leaf  is  fully  expanded,  with  its  four  main  divisions  and  numerous  leaflets  well 
spread.  In  the  figure  to  the  right  is  shown  the  same  leaf  after  it  has  been 
"shocked1'  by  a  sudden  touch,  or  by  sudden  heat,  or  in  some  other  way.  The 
leaflets  have  been  thrown  together  forward  and  upward  ;  the  four  main  divisions 
have  been  moved  together;  and  the  main  leaf-stalk  has  been  directed  sharply 
downward.  The  whole  change  has  very  much  reduced  the  surface  of  exposure.— 
After  Duchaktke. 


pass  plants,"  already  mentioned,  are  illustrations  of  this, 
the  leaves  standing  edgewise  and  receiving  on  their  surface 
the  less  intense  rays  of  light  (see  Figs.  5,  170).  In  the 
dry  regions  of  Australia  the  leaves  on  many  of  the  forest 
trees  and  shrubs  have  this  characteristic  edgewise  position, 
known  as  the  profile  position,  giving  to  the  foliage  a  very 
curious  appearance. 

Some  leaves  have  the  power  of  shifting  their  position 
according  to  their  needs,  directing  their  flat  surfaces  to- 
ward the  light,  or  more  or  less  inclining  them,  according 


Fig.  42.  The  telegraph  plant (Desmodium  ffyrans).  Each  leaf  is  made  up  of  three 
leaflets,  a  large  terminal  one,  and  a  pair  of  small  lateral  ones.  In  the  lowest  figure 
the  large  leaflets  are  spread  out  in  their  day  position  ;  in  the  central  figure  they  are 
turned  sharply  downward  in  their  night  position.  The  name  of  the  plant  refers  to 
the  peculiar  and  constant  motion  of  the  pair  of  lateral  leaflets,  each  one  of  which 
describes  a  curve  with  a  jerking  motion,  like  the  second-hand  of  a  watch,  as 
indicated  in  the  uppermost  figure. 


50 


PLANT   RELATIONS. 


to  the  danger.  Perhaps  the  most  completely  adapted 
leaves  of  this  kind  are  those  of  the  "sensitive  plants/' 
whose  leaves  respond  to  various  external  influences  by 
changing  their  positions.  The  common  sensitive  plant 
abounds  in  dry  regions,  and  may  be  taken  as  a  type  of 
such  plants  (see  Figs.  4,  41,  171).  The  leaves  are  divided 
into  very  numerous  small  leaflets,  sometimes  very  small, 
which  stretch  in  pairs  along  the  leaf  branches.  When 
drought  approaches,  some  of  the  pairs  of  leaflets  fold  to- 
gether, slightly  reduc- 
ing the  surface  expo- 
sure. As  the  drought 
continues,  more  leaflets 
fold  together,  then  still 
others,  until  finally  all 
the  leaflets  may  be 
folded  together,  and  the 


leaves   themsel 


ves    may 


Fig.  43.  Cotyledons  of  squash  seedling,  show- 
ing positions  in  light  (left  figure)  and  in 
darkness  (right  figure). — After  Atkinson. 


bend  against  the  stem. 
It  is  like  a  sailing  vessel 


gradually  taking  in  sail 
as  a  storm  approaches,  until  finally  nothing  is  exposed, 
and  the  vessel  weathers  the  storm  by  presenting  only  bare 
poles.  Sensitive  plants  can  thus  regulate  the  exposed  sur- 
face very  exactly  to  the  need. 

Such  motile  leaves  not  only  behave  in  this  manner  at  the 
coming  of  drought,  but  the  positions  of  the  leaflets  are 
shifted  throughout  the  day  in  reference  to  light,  and  at 
night  a  very  characteristic  position  is  assumed  (see  Figs.  2, 
3,  42),  once  called  a  "  sleeping  position/'  The  danger  from 
night  exposure  comes  from  the  radiation  of  heat  which 
occurs,  which  may  chill  the  leaves  to  the  danger  point. 
The  night  position  of  the  leaflets  of  Oxalis  has  been  re- 
ferred to  already  (see  §14).  Similar  changes  in  the  direc- 
tion of  the  leaf  planes  at  the  coming  of  night  may  be 
observed  in  most   of   the  Lefjuminosce,  even  the  common 


FOLIAGE   LEAVES  :    FUNCTION,    STRUCTURE,    ETC.        51 


VA 


B 


white  clover  displaying  it.  It  can  be  observed  that  the 
expanded  seed  leaves  (cotyledons)  of  many  young  germinat- 
ing plants  shift  their  positions  at  night  (see  Fig.  43),  often 
assuming  a  vertical  position  which  brings  them  in  contact 
with  one  another,  and  also  covers  the  stem  bud  (plumule). 

Certain  leaves  with  well-developed 
protective  structures  are  able  to  en- 
dure the  winter,  as  in  the  case  of 
the  so-called  evergreens.  In  the 
case  of  juniper,  however,  the  winter 
and  summer  positions  of  the  leaves 
are  quite  different  (see  Fig.  -14).  In 
the  winter  the  leaves  lie  close  against 
the  stem  and  overlap  one  another; 
while  with  the  coming  of  warmer 
conditions  they  become  widely 
spreading. 

39.  Protection  against  rain. — It  is 
also  necessary  for  leaves  to  avoid 
becoming  wet  by  rain.  If  the  water 
is  allowed  to  soak  in  there  is  danger 
of  filling  the  stomata  and  interfering 
with  the  air  exchanges.  Hence  it 
will  be  noticed  that  most  leaves  are 
able  to  shed  water,  partly  by  their 
positions,  partly  by  their  structure. 
In  many  plants  the  leaves  are  so  ar- 
ranged that  the  water  runs  off  towards  the  stem  and  so 
reaches  the  main  root  system  ;  in  other  plants  the  rain  is 
shed  outwards,  as  from  the  eaves  of  a  house. 

Some  of  the  structures  which  prevent  the  rain  from 
soaking  in  are  a  smooth  epidermis,  a  cuticle  layer,  waxy 
secretions,  felt-like  coverings,  etc.  Interesting  experi- 
ments may  be  performed  with  different  leaves  to  test  their 
power  of  shedding  water.  If  a  gentle  spray  of  water  is 
allowed  to  play  upon  different  plants,  it  will  be  observed 


Fig.  44.  Two  twigs  of  juni- 
per, showing  the  effect  of 
heat  and  cold  upon  the 
positions  of  the  leaves. 
The  ordinary  protected 
winter  position  of  the 
leaves  is  shown  by  A ; 
while  in  B,  in  response  to 
warmer  conditions,  the 
leaves  have  spread  apart 
and  have  become  freely  ex- 
posed.—After  Warming. 


52  PLANT   RELATIONS. 

that  the  water  glances  off  at  once  from  the  surfaces  of 
some  leaves,  runs  off  more  slowly  from  others,  and  may  be 
more  or  less  retained  by  others. 

In  this  same  connection  it  should  be  noticed  that  in 
most  horizontal  leaves  the  two  surfaces  differ  more  or  less 
in  appearance,,  the  upper  usually  being  smoother  than  the 
lower,  and  the  stomata  occurring  in  larger  numbers,  some- 
times exclusively,  upon  the  under  surface.  While  these 
differences  doubtless  have  a  more  important  meaning  than 
protection  against  wetting,  they  are  also  suggestive  in  this 
connection. 


CHAPTER   IV. 

SHOOTS. 

40.  General  characters. — The  term  shoot  is  used  to  include 
both  stem  and  leaves.  Among  the  lower  plants,  such  as 
the  algae  and  toadstools,  there  is  no  distinct  stem  and  leaf. 
In  such  plants  the  working  body  is  spoken  of  as  the  thallus, 
which  does  the  work  done  by  both  stem  and  leaf  in  the 
higher  plants.  These  two  kinds  of  work  are  separated  in 
the  higher  plants,  and  the  shoot  is  differentiated  into  stem 
and  leaves. 

41.  Life-relation. — In  seeking  to  discover  the  essential 
life-relation  of  the  stem,  it  is  evident  that  it  is  not  neces- 
sarily a  light-relation,  as  in  the  case  of  the  foliage  leaf, 
for  many  stems  are  subterranean.  Also,  in  general,  the 
stem  is  not  an  expanded  organ,  as  is  the  ordinary  foli- 
age leaf.  This  indicates  that  whatever  may  be  its  essential 
life-relation  it  has  little  to  do  with  exposure  of  surface. 
It  becomes  plain  that  the  stem  is  the  great  leaf-bearing 
organ,  and  that  its  life-relation  is  a  leaf-relation.  Often 
stems  branch,  and  this  increases  their  power  of  producing 
leaves. 

In  classifying  stems,  therefore,  it  seems  natural  to  use 
the  kind  of  leaves  they  bear.  From  this  standpoint  there 
are  three  prominent  kinds  of  stems  :  (1)  those  bearing  foli- 
age leaves  ;  (2)  those  bearing  scaly  leaves  ;  and  (3)  those 
bearing  floral  leaves.  There  are  some  peculiar  forms  of 
stems  which  do  not  bear  leaves  of  any  kind,  but  they  need 
not  be  included  in  this  general  view. 


54  PLANT   RELATIONS. 

A.     Stems  hearing  foliage  leaves. 

42.  General  character. — As  the  purpose  of  this  stem  is  to 
display  foliage  leaves,  and  as  it  has  been  discovered  that  the 
essential  life-relation  of  foliage  leaves  is  the  light-relation, 
it  follows  that  a  stem  of  this  type  must  be  able  to  relate  its 
leaves  to  light.  It  is,  therefore,  commonly  aerial,  and  that 
it  may  properly  display  the  leaves  it  is  generally  elongated, 
with  its  joints  {nodes)  bearing  the  leaves  well  separated  (see 
Pigs.  1,  4,  IS,  20). 

The  foliage-bearing  stem  is  generally  the  most  conspicu- 
ous part  of  the  plant  and  gives  style  to  the  whole  body. 
One's  impression  of  the  forms  of  most  plants  is  obtained 
from  the  foliage-bearing  stems.  Such  stems  have  great 
range  in  size  and  length  of  life,  from  minute  size  and  very 
short  life  to  huge  trees  which  may  endure  for  centuries. 
Branching  is  also  quite  a  feature  of  foliage-bearing  stems  ; 
and  when  it  occurs  it  is  evident  that  the  power  of  display- 
ing foliage  is  correspondingly  increased.  Certain  promi- 
nent types  of  foliage-bearing  stems  may  be  considered. 

43.  The  subterranean  type. — It  may  seem  strange  to  in- 
clude any  subterranean  stem  with  those  that  bear  foliage, 
as  such  a  stem  seems  to  be  away  from  any  light-relation. 
Ordinarily  subterranean  stems  send  foliage-bearing  branches 
above  the  surface,  and  such  stems  are  not  to  be  classed  as 
foliage-bearing  stems.  But  often  the  only  stem  possessed 
by  the  plant  is  subterranean,  and  no  branches  are  sent  to 
the  surface.  In  such  cases  only  foliage  leaves  appear  above 
ground,  and  they  come  directly  from  the  subterranean  stem. 
The  ordinary  ferns  furnish  a  conspicuous  illustration  of 
this  habit,  all  that  is  seen  of  them  above  ground  being  the 
characteristic  leaves,  the  commonly  called  "  stem  "  being 
only  the  petiole  of  the  leaf  (see  Figs.  45,  46,  144).  Many 
seed  plants  can  also  be  found  which  show  the  same  habit, 
especially  those  which  flower  early  in  the  spring.  This 
cannot  be    regarded  as  a  very  favorable  type  of  stem  for 


Fig.  45.  A  fern  (Aspidium),  showing  throe  large  branching  leaves  coming  from  a  hori- 
zontal subterranean  stem  (rootstock)  ;  growing  leaves  are  also  sliown,  which  are 
gradually  unrolling.  The  stem,  young  leaves,  and  petioles  of  the  large  leaves  are 
thickly  covered  with  protecting  hairs.  The  stem  gives  rise  to  numerous  small  roots 
from  its  lower  surface.  The  figure  marked  3  represents  the  under  surface  of  a 
portion  of  the  leaf,  showing  seven  groups  of  spore  eases;  at  5  is  represented  a 
section  through  one  of  these  groups,  showing  how  the  spore  cases  are  attached  and 
protected  by  a  tlap  ;  while  at  6  is  represented  a  single  spore  case  opening  and  dis- 
charging its  spores,  the  heavy  spring-like  ring  extending  along  the  back  and  over 
the  top.— After  Wossidlo. 
5 


66 


PLANT   KELATIONS. 


leaf  display,  and   as    a  rule  such    stems   do   not  produce 
many  foliage  leaves,  but  the  leaves  are  apt  to   be  large. 


Fig.  46.    A  common  fern,  showing  the  underground  stem  (rootstock),  which  sends  the 
few  large  foliage  leaves  above  the  surface.— After  Atkinson. 


The  subterranean  position  is  a  good  one,  however,  for 
purposes  of  protection  against  cold  or  drought,  and  when 
the  foliage  leaves  are  killed  new  ones  can  be  put  out  by 


SHOOTS. 


57 


the  protected  stem.  This  position  is  also  taken  advantage 
of  for  comparatively  safe  food  storage,  and  such  stems  are 
apt  to  become  more  or  less  thickened  and  distorted  by  this 
food  deposit. 

44.  The  procumbent  type.— In  this  case  the  main  body 
of  the  stem  lies  more  or  less  prostrate,  although  the  advanc- 
ing tip  is  usually  erect.  Such  stems  may  spread  in  all 
directions,  and  become  interwoven  into 
a  mat  or  carpet.  They  are  found 
especially  on  sterile  and  exposed  soil, 


Fig.  47.  A  strawberry  plant,  showing  a  runner  which  has  devel- 
oped a  new  plant,  which  in  turn  has  sent  out  another  run- 
ner.—After  Seubert. 


and  there  may  be  an  important  relation  between  this  fact  and 
their  habit,  as  there  may  not  be  sufficient  building  material 
for  erect  stems,  and  the  erect  position  might  result  in  too 
much  exposure  to  light,  or  heat,  or  wind,  etc.  Whatever 
may  be  the  cause  of  the  procumbent  habit,  it  has  its  advan- 
tages. As  compared  with  the  erect  stem,  there  is  economy 
of  building  material,  for  the  rigid  structures  to  enable  it  to 
stand  upright  are  not  necessary.  On  the  other  hand,  such 
a  stem  loses  in  its  power  to  display  leaves.  Instead  of 
being  free  to  put  out  its  leaves  in  every  direction,  one  side 
is  against  the  ground,  and  the  space  for  leaves  is  diminished 
at  least  one-half.  All  the  leaves  it  bears  are  necessarilv 
directed  towards  the  free  side  (see  Fig.  18). 

We  may  be  sure,  however,  that  any  disadvantage  com- 
ing from  this  unfavorable  position  for  leaf  display  is  over- 
balanced by  advantages  in  other  respects.     The  position  is 


58 


y^=r-r  mUa  £^<i 


PLANT   RELATIONS. 


certainly  one  of  protection,  and  it  has  a  further  advantage 
in  the  way  of  migration  and  vegetative  propagation.  As 
the  stem  advances  over  the  ground,  roots  strike  out  of  the 
nodes  into  the  soil.  In  this  way  fresh  anchorage  and  new 
soil  supplies  are  secured  ;  the  old  parts  of  the  stem  may 


Fig.  48.  Two  plants  of  a  saxifrage,  showing  rosette  habit,  and  also  the  numerous 
runners  sent  out  from  the  base,  which  strike  root  at  tip  and  produce  new  plants. 
—After  Kekner. 


die,  hut  the  newer  portions  have  their  soil  connection  and 
continue  to  live.  So  effective  is  this  habit  for  this  kind  of 
propagation  that  plants  with  erect  stems  often  make  use  of 
it,  sending  out  from  near  the  base  special  prostrate  branches, 
which  advance  over  the  ground  and  form  new  plants. 
A  very  familiar  illustration  is  furnished  by  the  straw- 
berry plant,  which  sends  out  peculiar  naked  "runners" 
to  strike  root  and  form  new  plants,  which  then  become 


-w~-£— <J 


*t  J^^- 1 


f^s^cr-r  (-^~^Ju£ 


SHOOTS. 


59 


independent  plants  by  the  dying  of  tlie  runners  (see  Figs. 
47,  48). 

45.  The  floating  type. — In  this  case  the  stems  are  sus- 
tained by  water.  Numerous  illustrations  can  be  found  in 
small  inland  lakes  and  slow-moving  streams  (see  Fig.  4'.)). 
Beneath  the  water  these  stems  often  seem  quite  erect,  but 


Fig.  49.    A  submerged  plant  (Cemtophyllum)  with  floating  stems,  showing  the  stem 
joints  bearing  finely  divided  leaves. 

when  taken  out  they  collapse,  lacking  the  buoyant  power 
of  the  water.  (J rowing  free  and  more  or  less  upright  in 
the  water,  Mdiey  seem  to  have  all  the  freedom  of  erect  stems 
in  displaying  foliage  leaves,  and  at  the  same  time  they 
are  not  called  upon  to  build  rigid  structures.  Economy 
of  building  material  and  entire  freedom  to  display  foliage 
would  seem  to  be  a  happy  combination  for  plants.  It  must 
be  noticed,  however,  that  another  very  important  condition 
is  introduced.  To  reach  the  Leaf  surfaces  the  light  must 
pass  through  the  water,  and  this  diminishes  its  intensity  so 


GO 


PLANT   RELATIONS. 


greatly  that  the  working  power  of  the  leaves  is  reduced. 
At  no  very  great  depth  of  water  a  limit  is  reached,  beyond 
which  the  light  is  no  longer  able  to  be  of  service  to  the 
leaves  in  their  work.     Hence  it  is  that  water  plants  are 

restricted  to  the  surface  of  the 
water,  or  to  shoal  places  ;  and  in 
such  places  vegetation  is  very 
abundant.  Water  is  so  serious 
an  impediment  to  light  that  very 
many  plants  bring  their  working 
leaves  to  the  surface  and  float 
them,  as  seen  in  water  lilies,  thus 
obtaining  light  of  undiminished 
intensity. 

■4G.  The  climbing  type. — Climb- 
ing stems  are  developed  especially 
in  the  tropics,  where  the  vegeta- 
tion is  so  dense  and  overshadow- 
ing that  many  stems  have  learned 
to  climb  upon  the  bodies  of  other 
plants,  and  so  spread  their  leaves 
in  better  light  (see  Figs.  50,  55, 
98,  212).  Great  woody  vines 
fairly  interlace  the  vegetation  of 
tropical  forests,  and  are  known 
as  "lianas,"  or  "lianes."  The 
same  habit  is  noticeable,  also,  in 
our  temperate  vegetation,  but  it 
is  by  no  means  so  extensively  dis- 
played as  in  the  tropics.  There 
are  a  good  many  forms  of  climb- 
ing stems.  Remembering  that 
the  habit  refers  to  one  stem  de- 
pending   upon    another    for 

are  ail  adjusted  to  face  the  light    mechanical  support,  we  may  in- 
anci  to  avoid  shading  one  an-  ..      -.  ,  .  *     ,, 

other  as  far  as  possible.  elude  many  hedge  plants  m  the 


SHOOTS. 


61 


list  of  climbers.  In  this  case  the  stems  are  too  weak  to 
stand  alone,  but  by  interlacing  with  one  another  they  may 
keep  an  upright  position.  There  are  stems,  also,  which 
climb  by  twining  about  their  support,  as  the  hop  vine  and 


Fk;.  51.     A  cluster  of  smilax,  showing  the  tendrils  which  enable  it  to  climb,  and  ah 
the  prickles.— After  Kkuner. 


morning  glory  ;  others  which  put  out  tendrils  to  grasp  the 
support  (see  Figs.  51,  52),  as  the  grapevine  and  star 
cucumber;  and  still  others  which  climb  by  sending  out 
suckers  to  act  as  holdfasts,  as  the  woodbine  (see  Figs.  53, 
54).     In  all  these  cases  there  is  an  attempt  to  reach  towards 


62 


PLANT    RELATIONS. 


the  light  without  developing  such  structures  in  the  stem 
as  would  enable  it  to  stand  upright. 

47.  The   erect   type.— This  type   seems   altogether   the 
best  adapted  for  the  proper  display  of  foliage  leaves.    Leaves 


.  52.    Passion-flower  vines  climbing  supports  by  means  of  tendrils,  which  may  be 

seen  more  or  less  extended  or  coiled.    The  two  types  of  leaves  upon  a  single  stem 
may  also  be  noted. 


can  be  sent  out  in  all  directions  and  carried  upward  to- 
wards the  light  ;  but  it  is  at  the  expense  of  developing  an 
elaborate  mechanical  system  to  enable  the  stem  to  retain 
this  position.  There  is  an  interesting  relation  between 
these  erect  bodies  and  zones  of  temperature.     At  high  alti- 


'  ~T 


SHOOTS. 


63 


Fig.  53.  Woodbine  (Ampelopsis)  in  a  deciduous  forest.  The  tree  trunks  are  almost 
covered  by  the  dense  masses  of  woodbine,  whose  leaves  are  adjusted  so  as  to  form 
compact  mosaics.  A  lower  stratum  of  vegetation  is  visible,  composed  of  shrubs 
and  tall  herbs,  showing  that  the  forest  is  somewhat  open. — After  Schimper. 


tildes  or  latitudes  the  subter- 
ranean and  prostrate  types  of 
foliage-bearing  stems  are  most 
common  ;  and  as  one  passes  to 
lower  altitudes  or  latitudes  the 
erect  stems  become  more  nu- 
merous and  more  lofty.  Among 
stems  of  the  erect  type  the  tree 
is  the  most  impressive,  and  it 
has  developed  into  a  great  vari- 
ety of  forms  or  "habits."  Any 
one  recognizes  the  great  differ- 
ence in  the  habits  of  the  pine 
and  the  elm  (see  Figs.  56, 
57,    58,    59),   and   many  of   our 


1'ic.  54.  A  portion  of  a  woodbine 
{Ampelopsis).  The  stem  tendrils 
have  attached  themselves  to  a 
smooth  wall  by  means  of  disk-like 
suckers.— After  Strasburger. 


Pig.  56.  A  tree  of  the  pine  type  (larch),  showing  the  continuous  central  shaft  and 
the  horizontal  branches,  which  tend  to  become  more  upright  towards  the  top  of 
the  tree.  The  general  outline  is  distinctly  conical.  The  larch  is  peculiar  among 
Buch  trees  in  periodically  shedding  its  leaves. 


Fig.  57.  A  pine  tree,  showing  the  central  shaft  and  also  the  bunching  of  the 
needle  leaves  toward  the  tips  of  the  branches  where  there  is  the  best  exposure 
to  light. 


SHOOTS. 


67 


common  trees  may  be  known,  even  at  a  distance,  by  their 
characteristic  habits  (see  Figs.  GO,  61,  62).  The  difficulty 
of  the  mechanical  problems  solved  by  these  huge  bodies 
is  very  great.  They  maintain  form  and  position  and  en- 
dure tremendous  pressure  and  strain. 


Fig.  58.  An  elm  in  its  winter  condition,  Bhowing  the  absence  of  a  continuous  central 
shaft,  the  main  stem  soon  breaking  up  into  branches,  and  giving  a  spreading  top. 
On  each  side  in  tbe  background  are  trees  of  the  pine  type,  Bhowing  the  centra] 
phaft  and  conical  outline, 


68 


PLANT   RELATIONS. 


48.  Relation  to  light. — As  stems  bearing  foliage  leaves 
hold  a  special  relation  to  light,  it  is  necessary  to  speak  of 
the  influence  of  light  upon  their  direction,  the  response  to 


An  elm  in  foliage,  showing  the  breaking  up  of  the  trunk  into  branches  and 
the  spreading  top. 

which  is  known  as  heliotropism,  already  referred  to  under 
foliage  leaves.  In  the  case  of  an  erect  stem  the  tendency 
is  to  grow  towards  the  source  of  light  (see  Figs.  1,  64). 


SHOOTS. 


69 


This  has  the  general  result  of  placing  the  leaf  blades  at 
right  angles  to  the  rays  of  light,  and  in  this  respect  the 
heliotropism  of  the  stem  aids  in  securing  a  favorable  leaf 
position  (see  Figs.  63,  63a).  Prostrate  stems  are  differently 
affected  by  the  light,  however,  being  directed  transversely 
to  the  rays  of  light.     The  same  is  true  of  many  foliage 


Fig.  60.    An  oak  in  its  winter  condition,  showing  the  wide  branching.    The 
directions  of  the  branches  have  been  determined  by  the  light-relations. 


branches,  as  maybe  seen  by  observing  almost  any  tree  in 
which  the  lower  branches  are  in  the  general  transverse  posi- 
tion. These  branches  generally  tend  to  turn  upwards  when 
they  are  beyond  the  region  of  shading.  Subterranean  stems 
are  also  mostly  horizontal,  but  they  are  out  of  the  influence 
of  light,  and  under  the  influence  of  gravity,  the  response  to 
which  is  known  as  geotropism,  which  guides  them  into  the 
transverse  position.     The  climbing  stem,  like  the  erect  one, 


70 


PLANT   KELATIONS. 


Fig.  61.    Cottonwoods,  in  winter  condition,  on  a  sand  dune,  showing  the  branching 
habit,  and  the  tendency  to  grow  in  groups. 


grows   towards   the   light,   while   floating   stems   may    be 
either  erect  or  transverse. 


B.     Stems  bearing  scale  leaves. 

40.  General  character. — A  scale  leaf  is  one  which  does 
not  serve  as  foliage,  as  it  does  not  develop  the  necessary 
chlorophyll.  This  means  that  it  does  not  need  such  an 
exposure  of  surface,  and  hence  scale  leaves  are  usually  much 
smaller,  and  certainly  are  more  inconspicuous  than  foliage 
leaves.  A  good  illustration  of  scale  leaves  is  furnished  by 
the  ordinary  scaly  buds  of  trees,  in  which  the  covering  of 
overlapping  scaly  leaves  is  very  conspicuous  (see  Fig.  65). 
As  there  is  no  development  of  chlorophyll  in  such  leaves, 


SHOOTS. 


71 


they  do  not  need  to  be  exposed  to  the  light.  Stems  bearing 
only  scale  leaves,  therefore,  hold  no  necessary  light-relation, 
and  may  be  subterranean  as  well  as  aerial.     For  the  same 


.Fig.  62.  A  group  of  weeping  birches,  showing  the  branching  habit  and  the  peculiar 
hanging  branchlets.  The  trunks  also  show  the  habit  of  birch  bark  in  peeling  off 
in  bands  around  the  stem. 


reason  scale  leaves  do  not  need  to  be  separated  from  one 
another,  but  may  overlap,  as  in  the  buds  referred  to. 

Sometimes  scale  leaves  occur  so  intermixed  with  foliage 


Fig.  63, 


.     Sunflowers  with  the  upper  part  of  the  stem  sharply  bent  towards  the  light, 
giving  the  leaves  better  exposure.— After  Schaffner. 


SHOOTS. 


73 


leaves  that  no  peculiar  stem  type  is  developed.  In  the 
pines  scale  leaves  are  found  abundantly  on  the  stems  which 
are  developed  for  foliage  purposes.  In  fact,  the  main  stem 
axes  of  pines  bear  only  scale  leaves,  while  short  spur-like 
branches  bear  the  characteristic  needles,  or  foliage  leaves, 
but  the  form  of  the 
stem  is  controlled 
by  the  needs  of  the 
foliage.  Some  very 
distinct  types  of 
scale-bearing  stems 
may  be  noted. 

50.  The  bud  type. 
— In  this  case  the 
nodes  bearing  the 
leaves  remain  close 
together,  not  sepa- 
rating, as  is  neces- 
sary in  ordinary 
foliage-bear  ing 
stems,  and  the 
leaves  overlap.  In 
a  stem  of  this  char- 
acter the  later  joints 
may  become  sepa- 
rated and  bear  foli- 
age  leaves,  so  that 

one  finds  scale  leaves  below  and  foliage  leaves  above  on 
the  same  stem  axis.  This  is  always  true  in  the  case  of 
branch  buds,  in  which  the  scale  leaves  serve  the  purpose 
of  protection,  and  are  aerial,  not  because  they  need  a 
light-relation,  but  because  they  are  protecting  young  foli- 
age leaves  which  do. 

Sometimes  the  scale  leaves  of  this  bud  type  of  stem  do 
not  serve  so  much  for  protection  as  for  food  storage,  and 
become  fleshy.     Ordinary  bulbs,  such  as  those  of  lilies,  etc., 


Fig.  63a.  Cotyledons  of  castor-oil  bean  ;  the  seedling 
to  the  left  showing  the  ordinary  position  of  the 
cotyledons,  the  one  to  the  right  showing  the  curva- 
ture of  the  stem  in  response  to  light  from  one 
side. — After  Atkinson. 


74 


PLANT    RELATIONS. 


Fig.  64.  An  araucarian  pine,  showing  the 
central  shaft,  and  the  regular  clusters  of 
branches  spreading  in  every  direction  and 
bearing  numerous  small  leaves.  The  low- 
ermost branches  extend  downwards  and 
are  the  largest,  while  those  above  become 
more  horizontal  and  smaller.  These  dif- 
ferences in  the  size  and  direction  of  the 
branches  secure  the  largest  light  expo- 
sure. 


are  of  this  character; 
and  as  the  main  pur- 
pose is  food  storage 
the  most  favorable 
position  is  a  subter- 
ranean one  (see  Fig. 
66).  Sometimes  such 
scale  leaves  become 
very  broad  and  not 
merely  overlap  but  en- 
wrap one  another,  as 
in  the  case  of  the 
onion. 

51.  The  tuber  type. 
— The  ordinary  potato 
may  be  taken  as  an  il- 
lustration (see  Fig. 
67) .  The  minute  scale 
leaves,  to  be  found  at 
the  "eyes"  of  the 
potato,  do  not  overlap, 
which  means  that  the 
stem  joints  are  farther 
apart  than  in  the  bud 
type.  The  whole  form 
of  the  stem  results 
from  its  use  as  a  place 
of  food  storage,  and 
hence  such  stems  are 
generally  subterra- 
nean. Food  storage, 
subterranean  position, 
and  reduced  scale 
leaves  are  facts  which 
seem  to  follow  each 
other  naturally. 


SHOOTS. 


75 


52.  The  rootstock  type. — This  is  prob- 
ably the  most  common  form  of  subter- 
ranean stem.  It  is  elongated,  as  are  foli- 
age stems,  and  hence  the  scale  leaves 
are  well  separated.  It  is  prominently 
used  for  food  storage,  and  is  also  admirably 
adapted  for  subterranean  migration  (see 
Fig.  68).  It  can  do  for  the  plant,  in  the 
way  of  migration,  what  prostrate  foliage- 
bearing  stems  do,  and  is  in  a  more  protected 
position.  Advancing  beneath  the  ground, 
it  sends  up  a  succession  of  branches 
to  the  surface.  It  is  a  very  efficient 
method  for  the  "spreading"  of  plants, 
and  is  extensively  used  by  grasses  in  cov- 
ering areas  and  forming  turf.  The  persist- 
ent continuance  of  the  worst  weeds  is  often 
due  to  this  habit  (see  Figs.  69,  70).     It 

is  impossible 
to  remove 
all  of  the 
indefinitely 
branch  i  n  g 


Pig.  65.  Branch  buds 
of  elm.  Three  buds 
(k)  with  their  over- 
lapping scales  are 
shown,  each  just 
above  the  scar  (b) 
of  an  old  leaf. — 
After  Behren-. 


Fig.  66.  A  bulb,  made  up  of  overlap- 
ping scales,  which  are  fleshy  on 
account  of  food  storage.  —  After 
Grat. 


rootstocks 

from  the  soil, 

and  any  fragments  that  remain 

are  able  to  send  up  fresh  crops 

of  aerial  branches. 

53.  Alternation  of  rest  and 
activity. — In  all  of  the  three 
stem  types  just  mentioned,  it 
is  important  to  note  that  they 
are  associated  with  a  remark- 
able alternation  between  rest 
and  vigorous  activity.  From 
the  branch  buds  the  new  leaves 


6 


PLANT  RELATIONS. 


emerge  with 
great  rapidity, 
and  trees  be- 
come covered 
with  new  foliage 
in  a  few  days. 
From  the  sub- 
terranean stems 
the  aerial  parts 
come  up  so 
speedily  that  the 
surface  of  the 
ground  seems  to 
be  covered  suddenly  with  young  vegetation.  This  sudden 
change  from  comparative  rest  to  great  activity  has  been 
well  spoken  of  as  the  "awakening  "  of  vegetation. 


Fig.  67.     A  potato  plant,  showing  the  subterranean  tubers 
After  Strasbtjrger. 


C.     Stems  hearing  floral  leaves. 


54.  The  flower. — The  so-called 
"flowers"  which  certain  plants 
produce  represent  another  type  of 
shoot,  being  stems  with  peculiar 
leaves.  8o  attractive  are  flowers 
that  they  have  been  very  much 
studied  ;  and  this  fact  has  led 
many  people  to  believe  that  flowers 
are  the  only  parts  of  plants  worth 
studying.  Aside  from  the  fact 
that  a  great  many  plants  do  not 
produce  flowers,  even  in  those 
that  do  the  flowers  are  connected 
with  only  one  of  the  plant  pro- 
cesses, that  of  reproduction. 
Every  one  knows  that  flowers  are 
exceedingly   variable,  and   names 


Fig.  m.  The  rootstock  of  Solo- 
mon's seal ;  from  the  under  side 
roots  are  developed  ;  and  on  the 
upper  side  are  seen  the  scars 
which  mark  the  positions  of  the 
successive  aerial  branches  which 
bear  the  leaves.  The  advanc- 
ing tip  is  protected  by  scales 
(forming  a  bud),  and  the  posi- 
tions of  previous  buds  are  in- 
dicated by  groups  of  ring-like 
scars  which  mark  the  attach- 
ment of  former  scales.  Advanc- 
ing in  front  and  dying  behind 
such  a  rootstock  may  give  rise 
to  an  indefinite  succession  of 
aerial  plants.— After  Gray. 


SHOOTS.  ?7 

have  been  given  to  every  kind  of  variation,  so  that  their 
study  is  often  not  much  more  than  learning  the  definitions 
of  names.  However,  if  we  seek  to  discover  the  life-rela- 
tions of  flowers  we  find  that  they  may  be  stated  very  simply. 
55.  Life-relations.— The  flower  is  to  produce  seed.  It 
must  not  only  put  itself  into  proper  relation  to  do  this,  but 


Fig.  69.  The  rootstock  of  a  rush  (Jiincus),  showing  how  it  advances  beneath  the 
ground  and  sends  above  the  surface  a  succession  of  branches.  The  breaking  up 
of  such  a  rootstock  only  results  in  so  many  separate  individuals.— After  Cowles. 

there  must  also  be  some  arrangement  for  putting  the  seeds 
into  proper  conditions  for  developing  new  plants.  In  the 
production  of  seed  it  is  necessary  for  the  flower  to  secure  a 
transfer  of  certain  yellowish,  powdery  bodies  which  it  pro- 
duces, known  as  pollen  or  pollen-grain s,  to  the  organ  in 
which  the  seeds  are  produced,  known  as  the  pistil.  This 
transfer  is  called  pollination.  One  of  the  important  things, 
therefore,  in  connection  with  the   flower,  is  for  it  to  put 


Fig.  70.     An  alpine  willow,  showing  a  strong  rootstock  developing  aerial  branches 
and  roots,  and  capable  of  long  life  and  extensive  migration.— After  Schimper. 


itself  into  such  relations  that 


Fig.  71.  A  flower  of  peony,  showing  the  four  sets  of 
floral  organs  :  k,  the  sepals,  together  called  the 
calyx  ;  c,  the  petals,  together  called  the  corolla  ; 
a,  the  numerous  stamens  ;  g,  the  two  carpels, 
which  contain  the  ovules.— After  Strasburger. 


it  may  secure  pollination. 
Besides  pollination, 
which  is  necessary 
to  the  production  of 
seeds,  there  must  he 
an  arrangement  for 
seed  distribution. 
It  is  always  well  for 
seeds  to  he  scattered, 
so  as  to  be  separated 
from  one  another 
and  from  the  parent 
plant.  The  two 
great  external  prob- 
lems in  connection 
w i t h  the  no w e r , 
therefore,   are  polli- 


SHOOTS. 


79 


nation  and  seed-distribution. 
It  is  necessary  to  call  attention 
to  certain  peculiar  features  of 
this  type  of  stem. 

5G.  Structures. — The  joints 
of  the  stem  do  not  spread 
apart,  so  that  the  peculiar 
leaves  are  kept  close  together, 
usually  forming  a  rosette-like 
cluster  (see  Fig.  71).  These 
leaves  are  of  four  kinds  :  the 
lowest  (outermost)  ones  (indi- 
vidually sepals,  collectively 
calyx)  mostly  resemble  small 
foliage  leaves  ;  the  next  higher 
(inner)  set  (individually petals, 
collectively  corolla)  are  usually 
the  most  conspicuous,  delicate 
in  texture  and  brightly  col- 
ored ;  the  third  set  (stamens) 
produces  the  pollen  ;  the 
highest  (innermost)  set  (car- 
pels) form  the  pistil  and  pro- 
duce the  ovules,  which  are  to 
become  seeds.  These  four  sets 
may  not  all  be  present  in  the 
same  flower  ;  the  members  of 
the  same  set  may  be  more  or 
less  blended  with  one  another, 
forming  tubes,  urns,  etc.  (see 
Figs.  72,  73,  74)  ;  or  the  dif- 
ferent members  may  be  modi- 
fied in  the  greatesl  variety  of 
ways. 

Another  peculiarity  of  this 
type  of  stem  is  that  when  the 


Fig.  72.  A  group  of  flowers  of  the  rose 
family.  The  one  at  the  top  (Poten- 
litta)  shows  three  broad  sepals, 
much  smaller  petals  alternating 
with  them,  a  group  of  stamens,  and 
a  large  receptacle  hearing  numer- 
ous small  carpels.  The  central  one 
(Alcln  milla)  shows  the  tips  of  two 
small  sepals,  three  larger  petals 
united  below,  stamens  arising  from 
the  rim  of  the  urn,  and  a  single  pe- 
culiar  pistil.  The  lowest  flower  (the 
common  apple)  shows  the  sepals, 
petals,  stamens,  and  three  styles, 
all  arising  from  the  ovary  part  of 
the  pistil.— After  Focke. 


80 


PLANT   RELATIONS. 


Fig 


A  flower  of  the  tobacco  plant :  a,  a  complete  flower,  showing  the  calyx  with 
its  sepals  blended  below,  the  funnelform  corolla  made  up  of  united  petals,  and  the 
stamens  just  showing  at  the  mouth  of  the  corolla  tube  ;  6,  a  corolla  tube  split  open 
and  showing  the  five  stamens  attached  to  it  near  the  base  ;  c,  a  pistil  made  up  of 
two  blended  carpels,  the  bulbous  base  (containing  the  ovules)  being  the  ovary,  the 
long  etalk-like  portion  the  style,  and  the  knob  at  the  top  the  stigma.— After 
Strasburger. 


last  set  of  floral  leaves  (carpels)  appear,  the  growth  of  the 
stem  in  length  is  checked  and  the  cluster  of  floral  leaves 


Fig.  74.  A  group  of  flower  forms  :  a,  a  flower  of  harebell,  showing  a  bell-shaped 
corolla  composed  of  live  petals  ;  h,  a  flower  of  phlox,  showing  a  tubular  corolla 
with  its  live  petals  distinct  above  and  sharply  spreading  :  c.n  flower  of  dead-nettle, 
showing  an  irregular  corolla  with  it*  five  petals  forming  two  lips  above  the  funnel- 
form  base  ;  d,  a  flower  of  toad-flax,  showing  a  two-lipped  corolla,  and  also  a  spur 
formed  by  the  base  of  the  corolla  ;  e,  a  flower  of  the  snapdragon,  showing  the  two 
lips  of  the  corolla  closed. — After  Gray. 


SHOOTS. 


81 


appears  to  be  upon  the  end  of  the  stem  axis.     It  is  usual, 
also,  for  the  short  stem  bearing  the  floral  leaves  to  broaden 


Fig.  75.  The  Star-of-Bethlehem  (OrnUhogalum),  showing  the  loose  cluster  of  flowers 
at  the  end  of  the  stem.  The  leaves  and  stem  arise  from  a  bulb,  which  produces  a 
cluster  of  roots  below.— After  Strasburgek. 

at  the  apex  and  form  what  is  called  a  receptacle,  upon  which 
the  close  set  floral  leaves  stand. 

Although  many  floral  stems  are  produced  singly,  it  is 


82 


PLANT   RELATION'S. 


very  common  for  them  to  branch,  so  that  the  flowers  appear 
in  clusters,  sometimes  loose  and  spray-like,  sometimes  com- 
pact  (see  Figs.   75,    76,   77).     For  example,  the  common 


Fig. 


A  flower  cluster  from  a  walnut  tree.— After  Strasburger. 


dandelion  "flower"  is  really  a  compact  head  of  flowers. 
All  of  this  branching  has  in  view  better  arrangements  for 
pollination  or  for  seed-distribution,  or  for  both. 

The  subject  of  pollination  and  seed-distribution  will  be 
considered  under  the  head  of  reproduction. 


SHOOTS. 


83 


STRUCTURE    AND    FUNCTION"    OF   THE    STEM. 

57.  Stem  structure. — The  aerial  foliage  stem  is  the  most 
favorable  for  studying  stem  structure,  as  it  is  not  distorted 
by  its  position  or  by  being  a  depository  for  food.  If  an 
active  twig  of  an  ordinary  woody  plant  be  cut  across,  it  will 


Fig. 


Flower  clusters  of  an  umbellifer  (JSvum).— After  Strasburger. 


be  seen  that  it  is  made  up  of  four  general  regions  (see  Fig. 
78):  (1)  an  outer  protecting  layer,  which  may  be  stripped 
off  as  a  thin  skin,  the  epidermis  ;  (2)  within  the  epider- 
mis a  zone,  generally  green,  the  cortex  ;  (3)  an  inner  zone 
of  wood  or  vessels,  known  as  the  vascular  region  ;  (4)  a 
central  pith. 

58.  Dicotyledons  and  Conifers. — Sometimes   the  vessels 


8± 


PLANT   RELATIONS. 


Fig.  78.  Section  across  a  young  twig  of  box 
elder,  showing  the  four  stem  regions  :  e, 
epidermis,  represented  by  the  heavy  bound- 
ing line  ;  c,  cortex  ;  w,  vascular  cylinder  ; 
j),  pith. 


are  arranged  in  a  hollow 
cylinder,  just  inside  of 
the  cortex,  leaving  Avhat 
is  called  pith  in  the 
center  (see  Fig.  78). 
Sometimes  the  pith  dis- 
appears in  older  stems  or 
parts  of  stems  and  leaves 
the  stem  hollow.  When 
the  vessels  are  arranged 
in  this  way  and  the  stem 
lives  more  than  a  year,  it 
can  increase  in  diameter 
by  adding  new  vessels 
outside    of    the    old.     In 

the  case  of  trees  these  additions  appear  in  cross-section  like 

a  series  of  concentric  rings,  and  as  there  is  usually  but  one 

growth  period  during  the  year,  they  are  often  called  annual 

rings  (see  Fig.  79),  and  the  age  of  a  tree  is  often  estimated 

by    counting    them. 

This  method  of  ascer- 
taining  the    age    of   a 

tree   is  not  absolutely 

certain,   as  there  may 

be    more   than    one 

growth  period  in  some 

years.     In  the  case  of 

trees    and    shrubs   the 

epidermis   is    replaced 

on   the  older  parts  by 

layers  of   cork,  which 

sometimes    becomes 

very  thick  and  makes 

up   the    outer  part  of 

what     is     commonly 

called  bark. 


Fig.  79.  Section  across  a  twig  of  box  elder  three 
years  old,  showing  three  annual  rings,  or  growth 
rings,  in  the  vascular  cylinder.  The  radiating 
lines  (m)  which  cross  the  vascular  region  (w)  rep- 
resent the  pith  rays,  the  principal  ones  extending 
from  the  pith  to  the  cortex  (c). 


SHOOTS. 


85 


Stems  which  increase  in  diameter  mostly  belong  to  the 
great  groups  called  Dicotyledons  and  Conifers.  To  the 
former  belong  most  of  our  common  trees,  such  as  maple, 
oak,  beech,  hickory,  etc.  (see  Figs.  58,  59,  GO,  Gl),  as 
well  as  the  great  majority  of  common  herbs;  to  the  latter 
belong  the  pines,  hemlocks,  etc.  (see  Figs.  5G,  57,  198 
to  201).  This  annual  increase  in  diameter  enables  the 
tree  to  put  out  an  increased  number  of  branches  and 
hence  foliage  leaves  each  year,  so 
that  its  capacity  for  leaf  work  be- 
comes greater  year  after  year.  A 
reason  for  this  is  that  the  stem  is 
conducting  important  food  sup- 
lilies  to  the  leaves,  and  if  it  in- 
creases in  diameter  it  can  conduct 
more  supplies  each  year  and  give 
work  to  more  leaves. 

59.  Monocotyledons. — In  other 
stems,  however,  the  vessels  are 
arranged  differently  in  the  central 
region.  Instead  of  forming  a  hol- 
low cylinder  enclosing  a  pith,  they 
are  scattered  through  the  central 
region,  as  may  be  seen  in  the  cross- 
section  of  a  corn-stalk    (see  Fig. 

80).  Such  stems  belong  mostly  to  a  great  group  of  plants 
known  as  Monocotyledons ,  to  which  belong  palms,  grasses, 
lilies,  etc.  For  the  most  part  such  stems  do  not  increase  in 
diameter,  hence  there  is  no  branching  and  no  increased 
foliage  from  year  to  year.  A  palm  well  illustrates  this 
habit,  with  its  columnar,  unbranching  trunk,  and  its  crown 
of  foliage  leaves,  which  are  about  the  same  in  number  from 
year  to  year  (see  Figs.  81,  82). 

60.  Ferns. — The  same  is  true  of  the  stems  of  most  fern- 
plants,  as  the  vessels  of  the  central  region  are  so  arranged 
that  there  can  be  no  diameter  increase,  though  the  ar- 


Fig.  80.  A  corn-stalk,  showing 
cross-section  and  longitudinal 
section.  The  dots  represent 
the  scattered  bundles  of  ves- 
sels, which  in  the  longitudinal 
section  are  seen  to  be  long 
liber-like  strands. 


Fig.  81.  A  date  palm,  showing  the  unbranched  columnar  trunk  covered  with  old  leaf 
bases,  and  with  a  cluster  of  huge  active  leaves  at  the  top,  only  the  lowest  portions 
of  which  are  shown.    Two  of  the  very  heavy  fruit  clusters  are  also  shown. 


SHOOTS. 


87 


rangement  is  very  different  from  that  found  in  Monocotyle- 
dons. It  will  be  noticed  how  similar  in  general  appearance 
is  the  habit  of  the  tree  fern  and  that  of  the  palm  (see  Fig. 
83). 

61.  Lower  plants. — In  the  case  of  moss-plants,  and  such 
alga3  and  fungi  as  develop  stems,  the  stems  are  very  much 


Fig.  82.     A  palm  of  the  palmetto  type  (fan  palm),  with  low  stem  and  a  crown  of  large 

leaves. 


simpler  in   construction,  but  they  servo   the  same  general 
purpose. 

(52.  Conduction  by  the  stem.— Aside  from  the  work  of 
producing  leaves  and  furnishing  mechanical  support,  the 
stem  is  a  great  conducting  region  of  the  plant.  This  sub- 
ject will  be  considered  in  Chapter  X.,  under  the  general 
head  of  "The  Nutrition  of  Plants," 


Fig.  83.    A  group  of  tropical  plants.     To  the  left  of  the  center  is  a  tree  fern,  with  its 
slender  columnar  stem  and  crown  of  large  leaves.    The  large-leaved  plants  to  the 


right  are  bananas  (monocotyledons). 


^ 


CHAPTER  V. 


ROOTS. 


63.  General  character. — The  root  is  a  third  prominent 
plant  organ,  and  it  presents  even  a  greater  variety  of  rela- 
tions than  leaf  or  stem.  In  whatever  relation  it  is  found 
it  is  either  an  absorbent  organ  or  a  holdfast,  and  very  often 
both.  For  such  work  no  light-relation  is  necessary,  as  in 
the  case  of  foliage  leaves  ;  and  there  is  no  leaf-relation,  as 
in  the  case  of  stems.  Roots  related  to  the  soil  may  be 
taken  as  an  illustration. 

It  is  evident  that  a  soil  root  anchors  the  plant  in  the 
soil,  and  also  absorbs  water  from  the  soil.  If  absorption  is 
considered,  it  is  further  evident  that  the  amount  of  it  will 
depend  in  some  measure  upon  the  amount  of  surface  which 
the  roots  expose  to  the  soil.  We  have  already  noticed  that 
the  foliage  leaf  has  the  same  problem  of  exjiosure,  and  it 
solves  it  by  becoming  an  expanded  organ.  The  question 
may  be  fairly  asked,  therefore,  why  are  not  roots  expanded 
organs  ?  The  receiving  of  rays  of  light,  and  the  absorbing 
of  water  are  very  different  in  their  demands.  In  the  former 
case  a  flat  surface  is  demanded,  in  the  latter  tubular  pro- 
cesses. The  increase  of  surface  in  the  root,  therefore,  is 
obtained  not  by  expanding  the  organ,  but  by  multiplying 
it.  Besides,  to  obtain  the  soil  water  the  roots  must  burrow 
in  every  direction,  and  must  send  out  their  delicate  thread- 
like branches  to  come  in  contact  with  as  much  soil  as  pos- 
sible. Furthermore,  in  soil  roots  absorption  is  not  the  only 
thing  to  consider,  for  the  roots  act  as  holdfasts  and  must 
grapple  the  soil.     This  is  certainly  done  far  more  effectively 


90 


PLANT    RELATIONS. 


by  numerous  thread-like  processes  spreading  in  every  direc- 
tion than  by  flat,  expanded  processes. 

It  should  also  be  noted  that  as  soil  roots  are  subterra- 
nean they  arc  used  often  for  the  storage  of  food,  as  in  the 
case  of  many  subterranean  stems.  Certain  prominent  root 
types  may  be  noted  as  follows  : 

(>4.   Soil  roots. — These  roots  push  into  the  ground  with 

great  energy, 
and  their  ab- 
s o r  1 ) i n g  sur- 
faces are  en- 
tirely covered. 
Only  the  young- 
est parts  of  a 
root  system 
absorb  actively, 
the  older  parts 
transporting 
the  absorbed 
material  to  the 
stem,  and  help- 
ing to  grip  the 
soil.  The  soil 
root  is  the  most 
common  root 
type,  being 
used  by  the  great  majority  of  seed  plants  and  fern  plants, 
and  among  the  moss  plants  the  very  simple  root-like  pro- 
cesses are  mostly  soil-related.  To  such  roots  the  water  of 
the  soil  presents  itself  either  as  free  water — that  is,  water 
that  can  be  drained  away — or  as  films  of  water  adhering  to 
each  soil  particle,  often  called  water  of  adhesion.  To  come 
in  contact  with  this  water,  not  only  does  the  root  system 
usually  branch  profusely  in  every  direction,  but  the  youngest 
branches  develop  abundant  absorbing  hairs,  or  root  hairs 
(see  Fig.  84),  which  crowd  in  among  the  soil  particles  and 


Fig.  84.  Root  tips  of  corn,  showing  root  hairs,  their  position 
in  reference  to  the  growing  tip,  and  the  effect  of  the 
surrounding  medium  upon  their  development  :  1,  in  soil  ; 
2.  in  air  ;  3,  in  water. 


ROOTS. 


91 


absorb  moisture  from  them. 
j 


Fio.  85.  Apparatus  to  show  the  influence 
of  water  (hydrotropism)  upon  the  direc- 
tion of  roots.  The  ends  (a)  of  the  hox 
have  hooks  for  hanging,  while  the  box 
proper  is  a  cylinder  or  trough  of  wire 
netting  and  is  filled  with  damp  sawdust. 
In  the  sawdust  are  planted  peas  (r/). 
whose  roots  (//.  '/,  k,  m) first  descend  until 
they  emerge  from  the  damp  sawdust,  but 
soon  turn  back  toward  it. — After  Sachs. 


By  these  root  hairs  the  ab- 
sorbing surface,  and  hence 
the  amount  of  absorption, 
is  greatly  increased.  Indi- 
vidual root  hairs  do  not  last 
very  long,  but  new  ones  are 
constantly  appearing  just 
behind  the  advancing  root 
tips,  and  the  old  ones  are 
as  constantly  disappearing. 
(1)  Geotropisin  and  hy- 
drotropism.— Many  outside 
influences  affect  roots  in 
the  direction  of  their 
growth,  and  as  soil  roots 
are  especially  favorable  for 
observing  these  influences, 
two  prominent  ones  may 
be  mentioned.  The  influ- 
ence of  gravity,  or  the  earth 
influence,  is  very  strong 
in  directing  the  soil   root. 


4  ,qj 


Fio. 


86.     A  raspberry  plant,  whose  stem  has  been  bent  down  to  the  soil  and  has 
"  struck  root.1''— After  Beal. 


92 


PLANT   RELATIONS. 


As  is  well  known,  when  a  seed  germinates  the  tip  that  is  to 
develop  the  root  turns  towards  the  earth,  even  if  it  has 
come  from  the  seed  in  some  other  direction.  This  response 
to  gravity  by  the  plant  is  known  as  geotropism.  Another 
directing  influence  is  moisture,  the  response  to  which  is 


■$££&?  y&% 


Fig.  87.  A  section  through  the  leaf  stalk  of  a  yellow  pond  lily  (Nvphar),  showing  the 
numerous  conspicuous  air  passages  (*•)  by  means  of  which  the  parts  under  water 
are  aerated  ;  h,  internal  hairs  projecting  into  the  air  passages  ;  r,  the  much 
reduced  and  comparatively  few  vascular  bundles. 

known  as  hydrotropism.  By  means  of  this  the  root  is  di- 
rected towards  the  most  favorable  water  supply  in  the  soil. 
Ordinarily,  geotropism  and  hydrotropism  direct  the  root 
in  the  same  general  way,  and  so  reinforce  each  other;  but 
the  following  experiment  may  be  arranged,  which  will 
separate  these  two  influences.  Bore  several  small  holes  in 
the  bottom  of  a  box  (such  as  a  cigar  box),  suspended  as  in- 
dicated in  Figure  85,  and  cover  the  bottom  with  blotting 
paper.      Pass  the   root  tips   of   several   germinated    seeds 


ROOTS. 


93 


through  the  holes,  so  that  the  seeds  rest  on  the  paper,  and 
the  root  tips  hang  through  the  holes.  If  the  paper  is  kept 
moist  germination  will  continue,  but  geotropism  will  direct 
the  root  tips  downwards,  and  hydrotropism  (the  moist 
paper)  will  direct  them  upwards.  In  this  way  they  will 
pursue  a  devious  course,  now  directed  by  one  influence 
and  now  by  the  other. 

If  a  root  system  be  examined  it  will  be  found  that  when 
there  is  a  main  axis  {tap 
root)  it  is  directed 
steadily  downwards, 
while  the  branches  are 
directed  differently. 
This  indicates  that  all 
parts  of  a  root  system 
are  not  alike  in  their 
response  to  these  influ- 
ences. Several  other 
influences  are  also  con- 
cerned in  directing  soil 
roots,  and  the  path  of 
any  root  branch  is  a 
result  of  all  of  them. 
How  variable  they  are 
may  be  seen  by  the 
numerous  directions  in 
which     the     branches 

travel,  and  the  whole  root  system  preserves  the  record  of 
these  numerous  paths. 

(2)  The  pull  on  the  stem. — Another  root  property  may 
be  noted  in  connection  with  the  soil  root,  namely  the  pull 
on  the  stem.  When  a  strawberry  runner  strikes  root  at 
tip  (see  Fig.  47),  the  roots,  after  they  obtain  anchorage  in 
the  soil,  pull  the  tip  a  little  beneath  the  surface,  as  if  they 
had  gripped  the  soil  and  then  slightly  contracted.  The 
same  thing    may  be   observed    in  the   process  known  as 


Fig.  88.  A  section  through  the  stem  of  a  water- 
wort  (Elutirte),  showing  the  remarkably  large 
and  regularly  arranged  air  passages  for  root 
aeration.  The  single  reduced  vascular  bundle 
is  central  and  connected  with  the  small  cor- 
tex by  thin  plates  of  cells  which  radiate  like 
the  spokes  of  a  wheel.— After  Schknck. 


94 


PLANT   RELATIONS. 


"layering,"  by  which  a  stem,  as  a  bramble,  is  bent  down 
and  covered  with  soil.  The  covered  joints  strike  root,  and 
the  pulling  follows  (see  Fig.  86).  A  very  plain  illustration 
of  this  pulling  by  roots  can  be  obtained  from  many  tuberous 
plants.  Tubers,  bulbs,  rootstocks,  etc.,  are  underground 
structures  which  have  been  observed  to  bury  themselves 
deeper  and  deeper  in  the  soil.    This  is  effected  by  the  young 


Fig.  89.     Section  through  the  loaf  of  a  quilhvort  (Isoetes),  Bhowing  the  four  large  air 
chambers  (a),  the  central  vascular  region  (b),  and  the  very  poorly  developed  cortex. 

roots  which  they  continue  to  put  forth.  These  roots  grip 
the  soil,  then  contract,  and  the  tuber  is  pulled  a  little  deeper. 
The  compact  tuber  known  as  the  Indian  turnip  ("  Jack-in- 
the-pulpit ")  has  been  found  to  bury  itself  very  deeply  and 
rapidly,  and  this  may  be  observed  by  transplanting  a  young 
and  vigorous  tuber  into  a  pot  of  loose  soil. 

(3)  Soil  dangers. — In  this  connection  certain  soil  dan- 
gers and  the  response  of  the  roots  should  be  noted.  The 
soil  may  become  poor  in  water  or  poor  in  certain  essential 
materials,  and  this  results  in  an  extension  of  the  root  sys- 


KOOTS. 


95 


tern,  as  if  seeking  for  water  and  the  essential  materials. 
Sometimes  the  root  system  becomes  remarkably  extensive, 
visiting  a  large  amount  of  soil  in  order  to  procure  the 
necessary  supplies.  Sometimes  the  soil  is  poor  in  heat,  and 
root  activity  is  interfered  with.  In  such  cases  it  is  very 
common  to  find  the  leaves 
massed  against  the  soil,  thus 
slightly  checking  the  loss  of 
heat. 

Most  soil  roots  also  need  free 
air,  and  when  water  covers  the 
soil  the  supply  is  cut  off.  In 
many  cases  there  is  some  way 
by  which  a  supply  of  free  air 
may  be  brought  down  into  the 
roots  from  the  parts  above 
water  ;  sometimes  by  large  air 
passages  in  leaves  and  stems 
(see  Figs.  87,  88,  89,  90)  ;  some- 
times by  developing  special  root 
structures  which  rise  above  the 
water  level,  as  prominently 
shown  by  the  cypress  in  the 
development  of  knees.  These 
knees  are  outgrowths  from  roots 
beneath  the  water  of  the  cypress 
swamp,  and  rise  above  the  water  level,  thus  reaching  the 
air  and  aerating  the  root  system  (see  Fig.  01).  It  has  been 
shown  that  if  the  water  rises  so  high  as  to  flood  the  knees 
for  any  length  of  time  the  trees  will  die.  but  it  does  not 
follow  that  this  is  the  chief  reason  for  their  development. 

Go.  Water  roots.— A  very  different  type  of  root  is  devel- 
oped if  it  is  exposed  to  free  water,  without  any  soil  relation. 
If  a  stem  is  floating,  clusters  of  whitish  thread-like  roots 
usually  put  out  from  it  and  dangle  in  the  water.  If  the  water 
level  sinks  so  as  to  bring  the  tips  of  these  roots  to  the  mucky 


Fig.  90.  Longitudinal  section 
through  a  young  quilhvort  leaf, 
showing  that  the  four  air  cham- 
bers shown  in  Fig.  89  are  not  con- 
tinuous passages,  but  that  there 
are  four  vertical  rows  of  promi- 
nent chambers.  The  plates  of 
cells  separating  the  chambers  in 
a  vertical  row  very  soon  become 
dead  and  full  of  air.  In  addition 
to  the  work  of  aeration  these  air 
chambers  are  very  serviceable  in 
enabling  the  leaves  to  float  when 
they  break  off  and  carry  the  com- 
paratively heavy  spore  cases. 


HOOTS. 


97 


Fig.  92.     A  tropical  aroid  (Anthurium),  showing  its  large  leaves,  and  bunches  of 

aerial  roots. 

soil  they  usually  do  not  penetrate  or  enter  into  any  soil  re- 
lation. Such  pure  water  roots  may  be  found  dangling  from 
the  under  surface  of  the  common  duck  weeds,  which  often 
cover  the  surface  of  stagnant  water  with  their  minute, 
green,  disk -like  bodies. 


98 


PLANT   RELATIONS. 


Plants  which  ordinarily  develop  soil  roots,  if  brought 
into  proper  water  relations,  may  develop  water  roots.  For 
instance,  willows  or  other  stream  bank  plants  may  be  so 
close  to  the  water  that  some  of  the  root  system  enters  it. 
In  such  cases  the  numerous  clustered  roots  show  their  water 


An  orchid,  showing  aerial  root6. 


character.  Sometimes  root  systems  developing  in  the  soil 
may  enter  tile  drains,  when  water  roots  will  develop  in  such 
clusters  as  to  choke  the  drain.  The  same  bunching  of  water 
roots  may  be  noticed  when  a  hyacinth  bulb  is  grown  in  a 
vessel  of  water. 

GG.  Air  roots. — In  certain  parts  of  the  tropics  the  air  is 
so  moist  that  it  is  possible  for  some  plants  to  obtain  sum- 


ROOTS. 


99 


cient  moisture  from  this  source,  without  any  soil-relation  or 
water-relation.  Among  these  plants  the  orchids  are  most 
notable,  and  they  may  be  observed  in  almost  any  green- 
house. Clinging  to  the  trunks  of  trees,  usually  imitated 
in  the  greenhouse  by  nests  of  sticks,  they  send  out  long 
roots  which  dangle  in  the  moist  air  (see  Figs.  93,  94). 
It  is  necessary  to  have  some  special  absorbing  and  condens- 
ing arrangement,  and  in  the 
orchids  this  is  usually  pro- 
vided by  the  development  of 
a  sponge-like  tissue  about  the 
root  known  as  the  velamen, 
which  greedily  absorbs  the 
moisture  of  the  air.  Examine 
also  Figs.  92,  95,  96,  97. 

67.  Clinging  roots. — These 
roots  are  developed  to  fasten 
the  plant  body  to  some  sup- 
port, and  do  no  work  of  ab- 
sorption (see  Fig.  98).  Very 
common  illustrations  may  be 
obtained  from  the  ivies,  the 
trumpet  creeper,  etc.  These 
roots  cling  to  various  supports, 
stone  walls,  tree  trunks,  etc., 
by  sending  minute  tendril- 
like branches  into  the  crevices 
develop  grasping  structures  extensively,  a  largo  majority 
of  them  being  anchored  to  rocks  or  to  some  rigid  support 
beneath  the  water,  and  their  bodies  floating  free.  The 
root-like  processes  by  which  this  anchorage  is  secured  are 
very  prominent  in  many  of  the  common  marine  sea-weeds 
(see  Fig.  L57). 

68.  Prop  roots. — Some  roots  are  developed  to  prop 
stems  or  wide-spreading  branches.  In  swampy  ground,  or 
in  tropical  forests,  it  is  very  common  to  find  the  base  of 


94.      An  orchid,   showing  aerial 
roots  and  thick  leaves. 


The   sea-weeds   (algae) 


Fio.  95. 


A  staghorn  fern  (Platy cerium),  an  aerial  plant  of  the  tropics.    About  it  is  a 
vine,  which  shows  the  leaves  adjusted  to  the  lighted  side. 


rhizophores  and  finely  divided  leaves. 


Fig. 


Live  oaks,  in  the  Gulf  States,  upon  which  are  growing  masses  of  long  moss 
or  black  moss  (Tillandsia),  a  common  aerial  plant. 


Fig.  98.  A  tropical  forest,  showing  the  cord-like  holdfasts  developed  by  an  epi- 
phyte, which  pass  around  the  tree  trunks  like  tightly  hound  ropes.— After 
Kerner, 


ROOTS 


103 


tree  trunks  buttressed  by  such  roots  which  extend  out  over 
and  beneath  the  surface,  and  divide  the  area  about  the  tree 
into  a  series  of  irregular  chambers  (see  Fig.  100).      Some- 


Fig.  99.     A  screw-pine  (Pandamts),  from  the    Indian  Ocean   region,   showing  the 
prominent  prop  roots  put  out  near  the  base. 

times  a  stem,  either  inclined  or  with  a  poorly  developed 
primary  root  system,  puts  out  prop  roots  which  support 
it,  as  in  the  screw-pine   (see  Fig.  99).     A  notable  case  is 
8 


106 


PLANT   RELATIONS. 


that  of  the  banyan  tree,  whose  wide-spreading  branches 
are  supported  by  pro})  roots,  which  are  sometimes  very 
numerous  (see  Fig.   101).     The  immense  banyans    usually 

illustrated  are 
especially  culti- 
vated as  sacred 
trees,  the  prop 
roots  being  as- 
sisted in  pene- 
trating the  soil. 
There  is  record 
of  such  a  tree  in 
Ceylon  with  350 
large  and  3,000 
small  prop  roots, 
able  to  cover  a 
village  of  100 
huts. 

09.  Parasites. 
— Besides  the 
roots  mentioned 
above,  certain 
plants  develop 
root-like  p  r  o- 
cesses  which  re- 
late ihemtohosts. 
A  host  is  a  liv- 
ing plant  or 
animal  upon 
which  so  m  e 
other  plant  or 
animal  is  living 
as  a  parasite. 
The  parasite  gets  its  supplies  from  the  host,  and  must  be 
related  to  it  properly.  If  the  parasite  grows  upon  the 
surface  of  its  host,  it  must  penetrate  the  body  to  obtain 


Fig.  102.  A  dodder  plant  parasitic  on  a  willow  twig.  The 
leafless  dodder  twines  about  the  willow,  and  sends  out 
sucking  processes  which  penetrate  and  absorb.— After 
Strasburger. 


ROOTS. 


107 


food  supplies. 
Therefore,  pro- 
cesses are  devel- 
oped which  pene- 
trate and  absorb. 
The  mistletoe  and 
dodder  are  seed- 
plants  which  have 
this  habit ,  and 
both  have  such 
processes  (see  Figs. 
102,  103).  This 
habit  is  much  more 
extensively  devel- 
oped, however,  in 
a  low  group  of 
plants  known  as 
the  fungi.  Many 
of  these  parasitic 
fungi  live  upon 
plants  and  animals, 
common  illustrations  being  the  mildews  of  lilac  leaves  and 
many  other  plants,  the  rust  of  wheat,  the  smut  of  corn,  etc. 

70.  Root  structure. 
— In  the  lowest  groups 
of  plants  (algae,  fungi, 
and  moss-plants)  true 
roots  are  not  formed, 
but  very  simple  struc- 
tures, generally  hair- 
like (see  Fig.  104).  In 
fern-plants  and  seed- 
plants,  however,  the 
root  is  a  complex 
structure,  so  different 
from  the  root-like  pro- 


Fig.  103.  A  section  showing  the  living  connection 
between  dodder  and  a  golden  rod  upon  which  it  is 
growing.  The  penetrating  and  absorbing  organ  (A) 
has  passed  through  the  cortex  (c),  the  vascular 
zone  (b),  and  is  disorganizing  the  pith  (p). 


Fig.  104.  Section  through  the  thallus  of  a  liver- 
wort (Marchantia),  Bhowing  the  hair-like  pro- 
cesses (rhizoids)  which  come  from  tlu'  under 
surface  and  act  as  roots  in  gripping  and  ab- 
sorbing. In  the  epidermis  of  the  upper  surface 
a  chimney-like  opening  is  seen,  leading  into 
a  chamber  containing  cells  with  chloroplasts. 


108 


PLANT   RELATIONS. 


cesses  of  the  lower  groups  that  it  is  regarded  as  the  only 
true  root.  It  is  quite  uniform  in  structure,  consisting  of  a 
tough  and  fibrous  central  axis  surrounded  by  a  region  of 
more  spongy  structure.  The  tough  axis  is  mostly  made 
up  of  vessels,  so  called  because  they 
conduct  material,  and  is  called  the 
vascular  axis.  The  outer  more  spongy 
region  is  the  cortex,  which  covers 
the  vascular  axis  like  a  thick  skin 
(see  Fig.  105). 

One  of  the  peculiarities  of  the 
root,  in  which  it  differs  from  the 
stem,  is  that  the  branches  come  from 
the  vascular  axis  and  burrow  through 
the  cortex,  so  that  when  the  latter 
is  peeled  off  the  branches  are  left 
attached  to  the  axis,  and  the  cortex 
shows  the  holes  through  which  they 
passed.  It  is  evident  that  when  such 
a  root  is  absorbing,  the  absorbed  ma- 
terial (water  with  various  materials 
in  solution)  is  received  into  the 
cortex,  through  which  it  must  pass 
to  the  vascular  axis  to  be  conducted 
to  the  stem. 

Another  peculiarity  of  the  root 
is  that  it  elongates  only  by  growth  at  the  tip,  while  the 
stem  usually  continues  to  elongate  some  distance  behind 
its  growing  tip.  In  the  soil  this  delicate  growing  tip  is 
protected  by  a  little  cap  of  cells,  known  as  the  root-cap 
(see  Fig.  105).  y? 


Fig 


A  longitudinal 
section  through  the  root 
tip  of  shepherd's  purse, 
showing  the  central  vas- 
cular axis  (p  ),  surrounded 
by  the  cortex  (p),  outside 
of  the  cortex  the  epi- 
dermis (e)  which  disap- 
pears in  the  older  parts  of 
the  root,  and  the  promi- 
nent root-cap  (c). 


CHAPTER  VI. 

REPRODUCTIVE  ORGANS. 


It  will  be  remembered  that  nutrition  and  reproduction 
are  the  two  great  functions  of  plants.  In  discussing 
foliage  leaves,  stems,  and  roots,  they  were  used  as  illustra- 
tions of  nutritive  organs,  so  far  as  their  external  relations 
are  concerned.  We  shall  now 
briefly  study  the  reproductive 
organs  from  the  same  point 
of  view,  not  describing  the 
processes  of  reproduction,  but 
some  of  the  external  relations. 

71.  Vegetative  multiplica- 
tion.— Among  the  very  lowest 
plants  no  special  organs  of 
reproduction  are  developed, 
but  most  plants  have  them. 
There  is  a  kind  of  reproduc- 
tion by  which  a  portion  of 
the  parent  body  is  set  apart  to 
produce  a  new  plant,  as  when 
a  strawberry  runner  produces 
a  new  strawberry  plant,  or 
when  a  willow  twig  or  a  grape 
cutting  is  planted  and  produces  new  plants,  or  when  a  potato 
tuber  (a  subterranean  stem)  produces  new  potato  plants,  or 
when  pieces  of  Begonia  leaves  are  used  to  start  new  Begonias. 
This  is  known  as  vegetative  multiplication,  a  kind  of  repro- 
duction which  does  not  use  special  reproductive  organs. 


Fig.  106.  A  group  of  spores  :  J.. 
spores  from  a  common  mold  (a 
fungus),  which  are  so  minute  and 
light  that  they  are  carried  about  by 
the  air  ;  B,  two  spores  from  a  com- 
mon alga  (Ilothrix),  which  can 
swim  by  means  of  the  hair-like 
processes;  C,  the  conspicuous  dotted 
cell  is  a  spore  developed  by  a  com- 
mon mildew  (i\  fungus),  which  is 
carried  about  by  currents  of  air. 


110 


PLANT   RELATIONS. 


72.  Spore  reproduction. — Besides  vegetative  multiplica- 
tion most  plants  develop  special  reproductive  bodies, 
known  as  spores,  and  this  kind  of  reproduction  is  known 
as  spore  reproduction.  These  spores  are  very  simple 
bodies,  but  have  the  power  of  producing  new  individuals. 
There  are  two  great  groups  of  spores,  differing  from  each 
other  not  at  all  in  their  powers,  but  in  the  method  of  their 
production   by  the   parent   plant.      One  kind  of  spore  is 

produced  by  dividing 
certain  organs  of  the 
parent  ;  in  the  other 
case  two  special  bodies 
of  the  parent  blend 
together  to  form  the 
spore.  Although  they 
are  both  spores,  for 
convenience  we  may 
call  the  first  kind 
spores  (see  Figs.  10G, 
109),  and  the  second 
kind  eggs  (see  Fig. 
107).*  The  two  special 
bodies  which  blend  to- 
gether to  form  an  egg 
are  called  gametes  (see 
Figs.  107,  108,  109).  These  terms  are  necessary  to  any 
discussion  of  the  external  relations.  Most  plants  develop 
both  spores  and  eggs,  but  they  are  not  always  equally  con- 
spicuous. Among  the  algae,  both  spores  and  eggs  are  prom- 
inent ;  among  certain  fungi  the  same  is  true,  but  many 
fungi  are  not  known  to  produce  eggs  ;  among  moss-plants 
the  spores  are  prominent  and  abundant,  but  the  egg  is 
concealed  and  not  generally  noticed.     What  has  been  said 

*  It  is  recognized  that  this  spore  is  really  a  fertilized  egg,  but  in 
the  absence  of  any  accurate  simple  word,  the  term  egg  is  used  for  con- 
venience. 


Fig.  107.  Fragments  of  a  common  alga  (Spi- 
rogyra).  Portions  of  two  threads  are  shown, 
which  have  been  joined  together  by  the  grow- 
ing of  connecting  tubes.  In  the  upper  thread 
four  cells  are  shown,  three  of  which  contain 
eggs  (z),  while  the  cell  marked  g,  and  its  mate 
of  the  other  thread  each  contain  a  gamete, 
the  lower  one  of  which  will  pass  through  the 
tube,  blend  with  the  upper  one,  and  form 
another  egg. 


REPRODUCTIVE    ORGANS. 


Ill 


of  the  moss-plants  is  still  more  true 
of  the  fern-plants ;  while  among 
the  seed-plants  certain  spores  (2J0I- 
len  grams)  are  conspicuous  (see 
Fig.  110),  but  the  eggs  can  be  ob- 
served only  by  special  manipulation 
in  the  laboratory.  Seeds  are  neither 
spores  nor  eggs,  but  peculiar  repro- 
ductive bodies  which  the  hidden 
egg  has  helped  to  produce. 

73.  Germination.  —  Spores  and 
eggs  are  expected  to  germinate ; 
that  is,  to  begin  the  development 
of  a  new  plant.  This  germination 
needs  certain  external  conditions, 
prominent  among  which  are  defi- 
nite amounts  of  heat,  moisture, 
and  oxygen,  and  sometimes  light. 
Conditions  of  germination  may  be 
observed  most  easily  in  connection 
with  seeds.  It  must  be  understood, 
however,   that   what    is    called    the 


Fig.  108.  A  portion  of  the 
body  of  a  common  alga 
( (JL(togo/uu?}t),  showing 
gametes  of  very  unequal  size 
and  activity  ;  a  very  large 
one  (0)  is  lying  in  a  globular 
cell,  and  a  very  small  one  is 
entering  the  cell,  another 
similar  one  (js)  being  just 
outside.  The  two  small 
gametes  have  hair-like  j no- 
cesses  and  can  swim  freely. 
The  small  and  large  gam- 
etes unite  and  form  an  egg. 


germination  of  seeds  is  something 

very  different  from  the   germination 
of   spores   and    eggs.      In  the  latter 


cases,  germination  includes "  the  v 


cry 


C 

Fig.  109.  A  group  of  swim- 
ming cells  :  A,  a  spore  of 
(Edogonium  (an  alga) ; 
B,  spores  of  Ulolhrix  (an 
alga) ;  C,  a  gamete  of 
Equteetum  (horse-tail  or 
scouring  rush). 


beginnings  of  the  young  plant.  \n 
the  case  of  a  seed,  germination  begun 
by  an  egg  has  been  checked,  and 
seed  germination  is  its  renewal.  In 
other  words,  an  egg  has  germinated 
and  produced  a  young  plant  called 
the  "embryo/5  and  the  germination 
of  the  seed  simply  consists  in  the 
continued  growth  and  the  escape  of 
this  embryo. 


112 


PLANT   RELATIONS. 


Fig.  110.  A  pollen  grain  (spore)  from  the 
pine,  which  develops  wings  (to)  to  assist 
in  its  transportation  by  currents  of  air. 


It  is  evident  that  for 
the  germination  of  seeds 
light  is  not  an  essential 
condition,  for  they  may 
germinate  in  the  light  or 
in  the  dark  ;  but  the  need 
of  heat,  moisture,  and 
oxygen  is  very  apparent. 
The  amount  of  heat  re- 
quired for  germination 
varies  widely  with  different 
seeds,  some  germinating 
at  much  lower  tempera- 
tures than  others.     Every 

kind  of  seed,  or  spore,  or  egg  has  a  special  temperature 

range,  below  which  and  above  which 

it  cannot  germinate.     The  two  limits 

of    the    range    may    be    called    the 

lowest   and    highest   points,   but   be- 
tween the  two  there  is  a  best  point 

of  temperature  for  germination.    The 

same  general  fact  is  true  in  reference 

to  the  moisture  supply. 

7-4.  Dispersal  of  reproductive  bodies. 

— Among  the  most  striking  external 

relations,    however,    are    those    con- 
nected with  the   dispersal  of  spores, 

gametes,   and   seeds.     Spores  and 

seeds  must  be  carried  away  from  the 

parent    plant,    and    separated    from 

each    other,     out    of    the    reach    of 

rivalry   for   nutritive    material  ;   and 

gametes    must    come    together    and 

blend  to  form  the  eggs.     Conspicuous 

among  the  means  of  transfer  are  the 

following. 


Fig.  111.  A  pod  of  fireweed 
(Epilobiwn)  opening  and 
exposing  its  plumed  seeds 
which  are  transported  by 
the  wind.— After  Beal. 


REPRODUCTIVE    ORGANS. 


113 


75.  Dispersal  by  locomotion. — The  common  method  of 
locomotion  is  by  means  of  movable  hairs  (cilia)  developed 
upon  the  reproductive  body,  which  propel  it  through  the 
water   (see  Fig.   109). 

Swimming  spores  are 
very   common   among  ,^  \  <^"; 

the  algae,  and  at  least 
one  of  the  gametes 
in  algae,  moss-plants, 
and  fern-plants  has 
the  power  of  swim- 
ming by  means  of 
cilia. 

76.  Dispersal  by 
water.  —  It  is  very 
common  for  repro- 
ductive bodies  to  be 
transported  by  cur- 
rents of  water.  The 
spores  of  many  water 
plants  of  all  groups, 
not  constructed  for 
locomotion,  are  thus 
floated  about.  This 
method  of  transfer  is 
also  very  common 
among  seeds.  Many 
seeds  are  buoyant,  or 
become  so  after  soak- 
ing in  water,  and 
may  be  carried  to 
great   distances   by 

currents.  For  this  reason  the  plants  growing  upon  the 
banks  or  flood-plains  of  streams  may  have  come  from  a 
wide  area.  Many  seeds  can  even  endure  prolonged  soak- 
ing in  sea-water,  and  then  germinate.     Darwin  estimated 


Fig.  112.  The  upper  figure  to  the  left  is  an  opening 
pod  of  fireweed  discharging  its  plumed  Beeds. 
The  lower  figure  represents  the  seed-like  fruits 
of  Clematis  with  their  long  tail-like  plumes.— 

After  Kkknkh. 


m 


PLANT    RELATIONS. 


that  at  least 
fourteen  per 
cent,  of  the 
seeds  of  any 
country  can  re- 
tain their  vital- 
ity in  sea-water 
for  twenty- 
eight  days.  At 
the  ordinary 
rate  of  move- 
ment of  ocean 
currents,  this 
length  of  time 
would  permit 
such  seeds  to 
be  transported 
over  a  thou- 
sand miles, 
thus  making 
possible   a   very   great   range   in   distribution. 

77.  Dispersal  of  spores  by  air. — This  is  one  of  the  most 
common  methods  of  transport- 
ing spores  and  seeds.  In  most 
cases  spores  are  sufficiently 
small  and  light  to  be  trans- 
ported by  the  gentlest  move- 
ments of  air.  Among  the 
fungi  this  is  a  very  common 
method  of  spore  dispersal  (see 
Fig.  10G),  and  it  is  extensively 
used  in  scattering  the  spores 
of  moss-plants,  fern-plants  (see 
Fig.  45),  and  seed-plants. 
Among  seed-plants  this  is  one 
method  of  pollination,  the 


Fig.  113.  A  ripe  dandelion  head,  showing  the  mass  of 
plumes,  a  few  seed-like  fruits  with  their  plumes  still 
attached  to  the  receptacle,  and  two  fallen  off.— After 
Kerner. 


Fig.  114.  Seed-like  fruits  of  Sevecio 
with  plumes  for  dispersal  by  air.— 
After  Kerner. 


REPRODUCTIVE    ORGANS 


115 


115.    A  winged  seed  of  Bignonia.— After  Strasburger. 

spores  called  pollen  grains  being  scattered  by  the  wind, 
and  occasionally 
falling  upon  the 
right  spot  for 
germination. 
With  such  an 
agent  of  transfer 
the  pollen  must 
be  very  light  and 
powdery,  and 
also  very  abun- 
dant, for  it  must 

Fig.  116.    Winged  fruit  of  maple.— After  Kerner. 

come    down   al- 
most like  rain  to  be  certain  of  reaching  the  right  places. 

Among  the  gynino- 
sperms  (pines,  hem- 
locks, etc.)  this  is  the 
exclusive  method  of 
pollination,  and  when  a 
pine  forest  is  shedding 
pollen  the  air  is  full  of 
the  spores,  which  may 
be  carried  to  a  great 
distance  before  being 

Fig.   117.      Winged    innt   of    Plelea.—  After  .  & 

Kerner.  deposited.        Occasional 


116 


PLANT   RELATIONS. 


Fig.  118.     Winged  fruit  of 
Ailanthus.— After  Ker- 

NER. 


reports  of  "showers  of  sulphur"  have 
arisen  from  an  especially  heavy  fall  of 
pollen  that  has  been  carried  far  from 
some  gymnosperm  forest.  In  the  case 
of  pines  and  their  near  relatives,  the 
pollen  spores  are  assisted  in  their  dis- 
persal through  the  air  by  developing  a 
pair  of  broad  wings  from  the  outer 
coat  of  the  spore  (see  Fig.  110).  This 
same  method  of  pollination — that  is, 
carrying  the  pollen  spores  by  currents 
of  air — is  also  used  by  many  mono- 
cotyledons, such  as  grasses  ;  and  by 
many  dicotyledons,  such  as  our  most 
common  forest  trees 
(oak,  hickory,  chest- 
nut, etc.). 

78.  Dispersal  of 
seeds  by  air. — Seeds 
are  very  rarely  light 
enough  to  be  carried 
by  currents  of  air 
without  some  special 
adaptation.  Wings 
and  plumes  of  very 
many  and  often  very 
beautiful  patterns 
are  exceedingly  com- 
mon in  connection 
with  seeds  or  seed- 
like fruits  (see  Figs. 
115,  110,  117,  118, 
L19).  Wings  are  de- 
veloped by  the  fruit 
of    maples    and   of 

1  Fig.  119.    Fruit  of  basswood  (Tiha),  showing  the 

ash,  and  by  the  Seeds  peculiar  wing  formed  by  a  leaf.— After  Kerner. 


REPRODUCTIVE   ORGANS. 


117 


Fig.  120.    A  common  tumbleweed  ( Cyclolomd). 


of  pine  and  catalpa.  Plumes  and  tnfts  of  hairs  are  devel- 
oped by  the  seed-like  fruits  of  dandelion,  thistle,  and  very 
many  of  their  relatives,  and  by  the  seeds  of  the  milkweed 
(see  Figs.  Ill,  112,  113,  114).  On  plains,  or  level  stretches, 
w here  w i  n  d  s  are 
strong,  a  curious 
habit  of  seed  dis- 
persal has  been  de- 
veloped by  certain 
plants  known  as 
"  tumbleweeds  "  or 
' (  fi  e  1  d  rollers." 
These  plants  are 
profusely  branching 
annuals  with  a  small 

Fig.  121.     The  3-valved  fruit  of  violet  discharging 
root      System     in     a  its  seeds.-After  Beal. 


118 


PLANT   RELATIONS. 


Fig.  122.  A  fruit  of  witch 
hazel  discharging  its 
seeds.— After  Beal. 


light  or  sandy  soil  (see  Fig.  120). 
When  the  work  of  the  season  is  over, 
and  the  absorbing  rootlets  have 
shriveled,  the  plant  is  easily  blown 
from  its  anchorage  by  a  gust  of  wind, 
and  is  trundled  along  the  surface  like 
a  light  wicker  ball,  the  ripe  seed  ves- 
sels dropping  their  seeds  by  the  way. 
In  case  of  an  obstruction,  such  as  a 
fence,  great  masses  of  these  tumble- 
weeds  may  often  be  seen  lodged 
against  the  windward  side. 

79.  Discharge  of  spores. — In  many 
plants  the  distribution  of  spores  and 
seeds  is  not  provided  for  by  any  of 

the  methods  just  mentioned,  but  the  vessels  containing 

them  are   so   constructed    that  they  are   discharged   with 

more  or  less  violence  and  are  some- 
what scattered. 

Many  spore  cases,  especially  those 

of  the  lower  plants,  burst  irregularly, 

and  with  sufficient  violence  to  throw 

out  spores.     In  the  liverworts  pecu- 
liar cells,  called  elaters  or  "jumpers," 

are  formed  among  the  spores,  and 

when   the  wall  of  the  spore  case  is 

ruptured  the  elaters  are   liberated, 

and   by  their  active  motion  assist  in 

discharging  the  spores. 

In  most  of  the  true  mosses  the 

spore  case  opens  by  pushing  off   a 

lid   at   the    apex,    which    exposes   a 

delicate  fringe  of  teeth  covering  the 

mouth  of  the  urn-like  case.     These 

teeth  bend  in  and  out  of  the  open 

spore  case  as  they  become  moist  or 


Fig.  123.  A  pod  of  wild  hean 
bursting,  the  two  valves 
violently  twisting  and  dis- 
charging the  seeds.— After 
Beal. 


REPRODUCTIVE    ORGANS. 


119 


dry,  and  are  of  considerable  service 
in  the  discharge  of  spores. 

In  the  common  ferns  a  heavy 
spring-like  ring  of  cells  encircles 
the  delicate-walled  spore  case. 
When  the  wall  becomes  dry  and 
comparatively  brittle  the  spring 
straightens  with  considerable  force, 
the  delicate  wall  is  suddenly  torn 
and  the  spores  are  discharged  (see 
Fig.  45). 

Even  in  the  case  of  the  pollen- 
spores  of  seed-plants,  a  special  layer 
of  the  wall  of  the  pollen-sac  usually 
develops  as  a  spring-like  layer,  which 
assists  in  opening  widely  the  sac 
when  the  wall  be- 
gins to  yield  along 
the  line  of  break- 
ing. 

80.  Discharge  of 
-While  seeds 


Pio.  125.  A  fruit  of 
b  e  g  g  :i  r  ticks, 
showing  the  two 

barbed      append- 
ages   which     lay 
hold  of  animals. 
—After  Beal. 
Q 


Fig.  124.  Fruits  of  Spanish 
needle,  showing  barbed  ap- 
pendages for  grappling. 
The  figure  to  the  left  is  one 
of  the  fruits  enlarged. — 
After  Kerner. 


are  generally  carried 
away  from  the  parent  plant  by  the  agency 
of  water  currents  or  air  currents,  as  al- 
ready noted,  or  by  animals,  in  some  in- 
stances there  is  a  mechanical  discharge 
provided  for  in  the  structure  of  the  seed- 
case.  In  such  plants  as  the  witch  hazel 
and  violet,  the  walls  of  the  seed-vessel 
press  upon  the  contained  seeds,  so  that 
when  rupture  occurs  the  seeds  are  pinched 
out,  as  a  moist  apple-seed  is  discharged 
by  being  pressed  between  the  thumb  and 
linger  (see  Figs.  121,  122).  In  the  touch- 
me-not  a  strain  is  developed  in  the  wall 
of  the  seed-vessel,  so  that  at  rupture  it 


120 


PLANT   KELATIONS. 


suddenly  curls  up  and  throws  the  seeds  (see  Fig.  123).  The 
squirting  cucumber  is  so  named  because  it  becomes  very 
much  distended  with  water,  which  is  finally  forcibly  ejected 
along  with  the  mass  of  seed.    An  "artillery  plant "  common 

in  cultivation  discharges  its 
seeds  with  considerable  vio- 
lence ;  while  the  detonations 
resulting  from  the  explosions 
of  the  seed-vessels  of  Hara 
crejyitans,  the  " r  monkey's  din- 
ner bell/'  are  often  remarked 
by  travelers  in  tropical 
forests. 

81.  Dispersal  of  seeds  by  animals. — Only  a  few  illustra- 
tions can  be  given  of  this  very  large  subject.  Water  birds 
are  great  carriers  of  seeds  which  are  contained  in  the  mud 
clinging  to  their  feet  and  legs.  This  mud  from  the  borders 
of  ponds  is  usually  completely  filled  with  seeds  and  spores 
of  various  plants.  One  has  no  concejotion  of  the  number 
until  they  are  actually  com- 


Fiu.  126.  The  fruit  of  carrot,  showing 
the  grappling  appendages.— After 
Beal. 


puted.  The  following  ex- 
tract from  Darwin's  Origin 
of  Species  illustrates  this 
point  : 


Fig.  127.    The  fruit  of  cocklebur,  showing 
the  grappling  appendages. —After  Beal. 


"I  took,  in  February,  three 
tablespoonfuls  of  mud  from  three 
different  points  beneath  water, 
on  the  edge  of  a  little  pond.  This  mud  when  dried  weighed  only  6f 
ounces  ;  I  kept  it  covered  up  in  my  study  for  six  months,  pulling  up 
and  counting  each  plant  as  it  grew  ;  the  plants  were  of  many  kinds, 
and  were  altogether  537  in  number  ;  and  yet  the  viscid  mud  was  all 
contained  in  a  breakfast  cup  !  " 

Water  birds  are  generally  high  and  strong  fliers,  and  the 
seeds  and  spores  may  thus  be  transported  to  the  margins  of 
distant  ponds  or  lakes,  and  so  very  widely  dispersed. 

in  many  cases  seeds  or  fruits  develop  grappling  append- 


REPRODUCTIVE    ORGANS. 


121 


ages  of  various  kinds,  which  lay  hold  of  animals  brushing 
past,  and  so  the  seeds  are  dispersed.  Common  illustrations 
are  Spanish  needles,  beggar  ticks,  stick  seeds,  burdock,  etc. 
Study  Figs.  124,  125,  126,  127,  128,  129,  130. 


Fie.  128.    Fruits  with  grappling  appendages.    That  to  the  left  is  agrimony  ;  that  to 
the  right  is  Galium. — After  Kerner. 

In  still  other  cases  the  fruit  becomes  pulpy,  and  attrac- 
tive as  food  to  certain  birds  or  mammals.  Many  of  the 
seeds  (such  as  those  of  grapes)  may  be  able  to  resist  the 
attacks  of  the  digestive  fluids  and  escape  from  the  alimen- 
tary tract  in  a  condition  to  germinate.  As  if  to  attract  the 
attention  of  fruit-eating  animals,  fleshy  fruits  usually 
become  brightly  col- 
ored when  ripe,  so  that 
they  are  plainly  seen 
in  contrast  with  the 
foliage. 

82.  Dispersal  of  pol- 
len spores  by  insects. — 
The  transfer  of  pollen, 
the   name    applied    to     FlG-  129-    Fruits  with  ?raPPlins  appendages. 

r  The  figure  to  the  left  is  cocklebur  ;  that  to  the 

certain  spores  oi  seed-        right  is  burdock.-After  kerner. 


122 


PLANT   EELATIONS. 


plants,  is  known  as  pollination,  and 
the  two  chief  agents  of  this  transfer 
are  currents  of  air  and  insects.  In 
§77  the  transfer  by  currents  of  air 
was  noted,  snch  plants  being  known 
as  anemophilous  plants.  Such  plants 
seldom  produce  what  are  generally 
recognized  as  true  flowers.  All  those 
seed-plants  which  produce  more  or 
less  showy  flowers,  however,  are  in 
some  way  related  to  the  visits  of 
insects  to  bring  about  pollination, 
and  are  known  as  entomophilous 
plants.  This  relation  between  in- 
sects and  flowers  is  so  important  and  so  extensive  that  it 
will  be  treated  in  a  separate  chapter. 


Fig.  130.  A  head  of  fruits  of 
burdock,  showing  the 
grappling  appendages. — 
After  Beal. 


CHAPTER  VII. 

FLOWERS  AND  INSECTS. 

83.  Insects  as  agents  of  pollination.— The  use  of  insects 
as  agents  of  pollen  transfer  is  very  extensive,  and  is  the  pre- 
vailing method  of  pollination  among  monocotyledons  and 
dicotyledons.  All  ordinary  flowers,  as  usually  recognized, 
are  related  in  some  way  to  pollination  by  insects,  but  it 
must  not  be  supposed  that  they  are  always  successful  in 
securing  it.  This  mutually  helpful  relation  between  flow- 
ers and  insects  is  a  very  wonderful  one,  and  in  some  cases 
it  has  become  so  intimate  that  they  cannot  exist  without 
each  other.  Flowers  have  been  modified  in  every  way  to  be 
adapted  to  insect  visits,  and  insects  have  been  variously 
adapted  to  flowers. 

84.  Self-pollination  and  cross-pollination.— The  advantage 
of  this  relation  to  the  flower  is  to  secure  pollination.  The 
pollen  may  be  transferred  to  the  carpel  of  its  own  flower, 
or  to  the  carpel  of  some  other  flower.  The  former  is  known 
as  self-pollination,  the  latter  as  cross-pollination.  In  the 
case  of  cross-pollination  the  two  flowers  concerned  may  be 
upon  the  same  plant,  or  upon  different  plants,  which  may 
be  quite  distant  from  one  another.  It  would  seem  that 
cross-pollination  is  the  preferred  method,  as  flowers  are  so 
commonly  arranged  to  secure  it. 

85.  Advantage  to  insects.— The  advantage  of  this  relation 
to  the  insect  is  to  secure  food.  This  the  flower  provides 
cither  in  the  form  of  nectar  or  pollen  ;  and  insects  visiting 
flowers  may  be  divided  roughly  into  the  two  groups  of 
nectar-feeding  insects,  represented  by  butterflies  and  moths, 


124  PLANT   RELATIONS. 

and  pollen-feeding  insects,  represented  by  the  numerous 
bees  and  wasps.  When  pollen  is  provided  as  food,  the 
amount  of  it  is  far  in  excess  of  the  needs  of  pollination. 
The  presence  of  these  supplies  of  food  is  made  known  to 
the  insect  by  the  display  of  color  in  connection  with  the 
flowers,  by  odor,  or  by  form.  It  should,  be  said  that  the 
attraction  of  insects  by  color  has  been  doubted  recently,  as 
certain  experiments  have  suggested  that  some  of  the  com- 
mon flower-visiting  insects  are  color-blind,  but  remarkably 
keen-scented.  However  this  may  be  for  some  insects,  it 
seems  to  be  sufficiently  established  that  many  insects  rec- 
ognize their  feeding  ground  by  the  display  of  color. 

86.  Suitable  and  unsuitable  insects. — It  is  evident  that 
all  insects  desiring  nectar  or  pollen  for  food  are  not  suit- 
able for  the  work  of  pollination.  For  instance,  the  ordi- 
nary ants  are  fond  of  such  food,  but  as  they  walk  from  plant 
to  plant  the  pollen  dusted  upon  them  is  in  great  danger  of 
being  brushed  off  and  lost.  The  most  favorable  insect  is 
the  flying  one,  that  can  pass  from  flower  to  flower  through 
the  air.  It  will  be  seen,  therefore,  that  the  flower  must  not 
only  secure  the  visits  of  suitable  insects,  but  must  guard 
against  the  depredations  of  unsuitable  ones. 

87.  Danger  of  self-pollination.— There  is  still  another 
problem  which  insect-pollinating  flowers  must  solve.  If 
cross-pollination  is  more  advantageous  to  the  plant  than 
self-pollination,  the  latter  should  be  prevented  so  far  as 
possible.  As  the  stamens  and  carpels  are  usually  close  to- 
gether in  the  same  flower,  the  clanger  of  self-pollination  is 
constantly  present  in  many  flowers.  In  those  plants  which 
have  stamen-producing  flowers  upon  one  plant  and  carpel- 
producing  flowers  upon  another,  there  is  no  such  danger. 

88.  Problems  of  pollination. — In  most  insect-pollinating 
flowers,  therefore,  there  are  three  problems  :  (1)  to  prevent 
self-pollination,  (2)  to  secure  the  visits  of  suitable  insects, 
and  (3)  to  ward  off  the  visits  of  unsuitable  insects.  It 
must  not  be  supposed  that  flowers  are  uniformly  successful 


FLOWERS   AND    INSECTS. 


125 


in  solving  these  problems.     They  often  fail,  but  succeed 
often  enough  to  make  the  effort  worth  while. 

89.  Preventing  self-pollination. — It  is  evident  that  this 
danger  arises  only  in  those  flowers  in  which  the  stamens 
and  carpels  are  associ- 
ated, but  their  se jura- 
tion in  different  flowers 
may  be  considered  as 
one  method  of  prevent- 
ing self-pollination.  In 
order  to  understand  the 
various  arrangements  to 
be  considered,  it  is  nec- 
essary to  explain  that 
the  carpel  does  not  re- 
ceive the  pollen  indif- 
ferently over  its  whole 
surface.  There  is  one 
definite  region  organ- 
ized, known  as  the 
stigma,  upon  which  the 
pollen  must  be  deposited 
if  it  is  to  do  its  work. 
Usually  this  is  at  the 
most  projecting  point 
of  the  carpel,  very  often 
at  the  end  of  a  stalk- 
like  prolongation  from 
the  ovary  (the  bulbous 
part  of  the  carpel), 
known  as  the  style; 
sometimes  it  may  run  down  one  side  of  the  style.  When 
the  stigma  is  ready  to  receive  pollen  it  has  upon  it  a 
sweetish,  sticky  fluid,  which  holds  and  feeds  the  pollen. 
In  this  condition  the  stigma  is  said  to  be  mature  ;  and  the 
pollen  is  mature  when  it  is  shedding,  that  is,  ready  to  fall 


Fig.  131.  Parts  of  the  flower  of  rose  acacia 
(Bobiniahispida).  In  1  the  keel  is  shown  pro- 
jecting from  the  hairy  calyx,  the  other  more 
showy  parts  of  the  corolla  having  been  re- 
moved. Within  the  keel  are  the  stamens 
and  the  carpel,  as  seen  in  3.  The  keel  forms 
the  natural  landing  place  of  a  visiting  bee, 
whose  weight  depresses  the  keel  and  causes 
the  tip  of  the  style  to  protrude,  as  shown  in 
2.  This  style  tip  bears  pollen  upon  it, 
caught  among  the  hairs,  seen  in  3.  and  as  it 
strikes  the  body  of  the  bee  some  pollen  is 
brushed  off.  If  the  bee  has  previously  visited 
another  flower  and  received  some  pollen,  it 
will  be  seen  that  the  stigma,  at  the  very  tip 
of  the  style,  striking  the  body  first,  will  very 
probably  receive  some  of  it.  The  nectar  pit 
is  shown  in  3,  at  the  base  of  the  uppermost 
stamen.— After  Gkay. 


126 


PLANT    RELATIONS. 


out  of  the  pollen-sacs  or  to  be  removed  from  them.  The 
devices  used  by  flowers  containing  both  stamens  and  carpels 
to  prevent  self-pollination  are  very  numerous,  but  most 
of  them  may  be  included  under  the  three  following  heads  : 
(1)  Pus  if  ion.  —  In  these  cases  the 
pollen  and  stigma  are  ready  at  the  same 
time,  but  their  position  in  reference  to 
each  other,  or  in  reference  to  some  con- 
formation of  the  flower,  makes  it  un- 
likely that  the  pollen  will  fall  upon  the 
stigma.  The  stigma  may  be  placed 
above  or  beyond  the  pollen  sacs,  or  the 
two  may  be  separated  by  some  mechan- 
ical obstruction,  resulting  in  much  of 
the  irregularity  of  flowers. 

In  the  flowers  of  the  rose  acacia  and 
its  relatives,  the  several  stamens  and 
the  single  carpel  are  in  a  cluster,  en- 
closed in  the  keel  of  the  flower.  The 
stigma  is  at  the  summit  of  the  style, 
and  projects  somewhat  beyond  the 
pollen-sacs  shedding  pollen.  Also  there 
is  often  a  rosette  of  hairs,  or  bristles, 
just  beneath  the  stigma,  which  acts  as 
a  barrier  to  the  pollen  (see  Fig.  131). 

In  the  iris,  or  common  flag,  each 
stamen  is  in  a  sort  of  pocket  between 
the  petal  and  the  petal-like  style,  while 
the  stigmatic  surface  is  on  the  top  of  a 
flap,  or  shelf,  which  the  style  sends  out 
as  a  roof  to  the  pocket.  With  such  an 
arrangement,  it  would  seem  impossible 
for  the  pollen  to  reach  the  stigma  un- 
aided (see  Fig.  132). 

In  the  orchids,  remarkable  for  their 
strange  and  beautiful  flowers,  there  are 


Fig.  132.    A  portion  of 

the  flower  of  an  iris, 
or  flag.  The  single 
stamen  shown  is 
standing  between  the 
petal  to  the  right  and 
the  petal-like  style  to 
the  left.  Near  the 
top  of  this  style  the 
stigmatic  shelf  is 
seen  extending  to  the 
right,  which  must 
receive  the  pollen 
upon  its  upper  sur- 
face. The  nectar 
pit  is  at  the  junc- 
tion of  the  petal  and 
stamen.  While  ob- 
taining the  nectar  the 
insect  brushes  the 
pollen-bearing  part 
of  the  stamen,  and 
pollen  is  lodged  upon 
its  body.  In  visiting 
the  next  flower  and 
entering  the  stamen 
chamber  the  stig- 
matic shelf  is  apt  to 
be  brushed.— After 
Gray. 


FLOWERS   AND    INSECTS. 


127 


usually  two  pollen-sacs,  and  stretched  between  them  is  the 
stigmatic  surface.  In  this  case,  however,  the  pollen  grains 
are  not  dry  and  powdery,  but  cling  together  in  a  mass,  and 
cannot  escape  from  the  sac  without  being  pulled  out  (see 
Fig.  133).  The  same  sort  of  pollen  is  developed  by  the 
milkweeds. 

(2)  Consecutive  maturity. — In  these  cases  the  pollen  and 


Fig.  133.  A  flower  of  an  orchid  (Habenaria).  At  1  the  complete  flower  is  shown, 
with  three  sepals  behind,  and  three  petals  in  front,  the  lowest  one  of  which  has 
developed  a  long  strap-shaped  portion,  and  a  still  longer  spur  portion,  the  opening 
to  which  is  seen  at  the  base  of  the  strap.  At  the  bottom  of  this  long  spur  is  the 
nectar,  which  is  reached  by  the  long  proboscis  of  a  moth.  The  two  pollen  sacs  of 
the  single  stamen  are  seen  in  the  centre  of  the  flower,  diverging  downwards,  and 
between  them  stretches  the  stigma  surface.  The  relation  between  pollen  sacs  and 
Btigma  surface  is  more  clearly  shown  in  2.  Within  each  pollen  sac  is  a  mass  of 
sticky  pollen,  ending  below  in  a  sticky  disk,  which  may  be  seen  in  1  and  'J.  When 
the  moth  thrusts  his  proboscis  into  the  nectar  tube,  his  head  is  againsl  the  Btig- 
matic  surface  and  also  againsl  the  disks.  When  be  removes  his  head  the  disks 
stick  fast  and  the  pollen  masses  are  dragged  out.  Tn  :}  a  pollen  mass  (a)  is 
shown  sticking  to  each  eye  of  a  moth.  Upon  visiting  another  Sower  these  pollen 
masses  are  thrust  against  the  stigmatic  surface  and  pollination  is  effected.— After 
Graf. 


128 


PLANT   RELATIONS. 


stigma  of  the  same  flower  are  not  mature  at  the  same  time. 
It  is  evideut  that  this  is  a  very  effective  method  of  prevent- 
ing self-pollination.  When  the  pollen  is  being  shed  the 
stigma  is  not  ready  to  receive,  or  when  the  stigma  is  ready 
to  receive  the  pollen  is  not  ready  to  be  shed.  In  some 
cases  the  pollen  is  ready  first,  in  other  cases  the  stigma, 
the  former  condition  being  called  protandry,  the  latter 
protogyny.     This  is  a  very  common  method  of  preventing 

self-pollination,  and  is  com- 
monly not  associated  with 
irregularity. 

The  ordinary  tigwort  may 
be  taken  as  an  example  of 
protogyny.  When  the  flowers 
first  open,  the  style,  bearing 
the  stigma  at  its  tip,  is  found 
protruding  from  the  urn-like 
flower,  while  the  four 
stamens  are  curved  down 
into  the  tube,  and  not  ready 
to  shed  their  pollen.  At 
some  later  time  the  style 
bearing  the  stigma  wilts, 
and  the  stamens  straighten 
up  and  protrude  from  the  tube.  In  this  way,  first  the 
receptive  stigma,  and  afterwards  the  shedding  pollen-sacs, 
occupy  the  same  position. 

Protandry  is  even  more  common,  and  many  illustrations 
can  be  obtained.  For  example,  the  showy  flowers  of  the 
common  fireweed,  or  great  willow  herb,  when  first  opened 
display  their  eight  shedding  stamens  prominently,  the  style 
being  sharply  curved  downward  and  backward,  carrying 
the  four  stigma  lobes  well  out  of  the  way.  Later,  the 
stamens  bend  away,  and  the  style  straightens  up  and  ex- 
poses its  stigma  lobes,  now  receptive  (see  Fig.  134). 

(3)  Difference  in  pollen. — In  these   cases  there  are  at 


Fig.  134.  Flowers  of  fireweed  (Epi- 
lobivm),  showing  protandry.  In  1  the 
stamens  are  thrust  forward,  and  the 
style  is  sharply  turned  downward  and 
backward.  In  2  the  style  is  thrust 
forward,  with  its  stigmatic  branches 
spread.  An  insect  in  passing  from  1 
to  2  will  almost  certainly  transfer  pol- 
len from  the  stamens  of  1  to  the  stig- 
mas of  2.— After  Gray. 


FLOWERS   AND    INSECTS. 


129 


least  two  forms  of  flowers,  which  differ  from  one  another 
in  the  relative  lengths  of  their  stamens  and  styles.  In  the 
accompanying  illustrations  of  Houstonia  (see  Fig.  135)  it 
is  to  be  noticed  that  in  one  flower  the  stamens  are  short 
and  included  in  the  tube,  and  the  style  is  long  and  pro- 
jecting, with  the  four  stigmas  exposed  well  above  the 
tube.  In  the 
other  flower  the 
relative  lengths 
are  exactly  re- 
versed,  the 
style  being 
short  and  in- 
cluded in  the 
tube,  and  the 
stamens  long 
and  projecting. 
It  appears  that 
the  pollen  from 
the  short  sta- 
mens is  most 
effective  upon 
the  stigmas  of 
the  short  styles, 
and  that  the 
pollen  from  the 
long  stamens  is 
most     effective 


Fig.  135.  Flowers  of  Houstonia,  showing  two  forms  of 
flowers.  In  1  there  are  short  stamens  and  a  long  style  ; 
in  2  long  stamens  and  short  style.  An  insect  visiting  1 
will  receive  a  band  of  pollen  about  the  front  part  of  its 
body  ;  upon  visiting  2  this  band  will  nil)  against  the 
stigmas,  and  a  fresh  pollen  band  will  be  received  upon 
the  hinder  part  of  the  body,  which,  upon  visiting  another 
flower  like  No.  1,  will  brush  against  the  stigmas.— 
. ,  ,  .  After  Gray. 

upon  the  stig- 
mas of  the  long  styles  ;  and  iis  short  stamens  and  long 
styles,  or  long  stamens  and  short  styles,  are  associated  in 
the  same  flower,  the  pollen  must  be  transferred  to  sonic 
other  flower  to  And  its  appropriate  stigma,  This  means 
that  there  is  a  difference  between  the  pollen  of  the  short 
stamens  and  that  of  the  long  ones. 

In  some  cases  there  are  three  forms  of  flowers,  as  in  one 


130 


PLANT   RELATIONS. 


of  the  common  loosestrifes.  Each  flower  has  stamens  of 
two  lengths,  which,  with  the  style,  makes  possible  three 
combinations.  One  flower  has  short  stamens,  middle-length 
stamens,  and  long  style  ;  another  has  short  stamens,  middle- 
length  style,  and  long  stamens  ;  the  third  has  short  style, 
middle-length  stamens,  and  long  stamens.  In  these  cases 
also  the  stigmas  are  intended  to  receive  pollen  from  stamens 


Fig.  136.  Yucca  and  Pronuba.  In  the  lower  figure  to  the  right  an  opened  flower 
shows  the  pendent  ovary  with  the  stigma  region  at  its  apex.  The  upper  figure  to 
the  right  shows  the  position  of  Pronuba  when  collecting  pollen.  The  figure  to  the 
left  represents  a  cluster  of  capsules  of  Yucca,  which  shows  the  perforations  made 
by  the  larvae  of  Pronuba  in  escaping.— After  Riley  and  Trelease. 


of  their  own  length,  and  a  transfer  of  pollen  from  flower  to 
flower  is  necessary. 

90.  Self-pollination. — In  considering  these  three  general 
methods  of  preventing  self-pollination,  it  must  not  be  sup- 
posed that  self-pollination  is  never  provided  for.  It  is  pro- 
vided for  more  extensively  than  was  once  supposed.  It  is 
found  that  many  plants,  such  as  violets,  in  addition  to  the 
usual  showy,  insect-pollinated  flowers,  produce  flowers  that 
are  not  at  all  showy,  in  fact  do  not  open,  and  are  often  not 
prominently  placed.  The  fact  that  these  flowers  are  often 
closed   has    suggested   for   them   the    name   cleistogamous 


FLOWERS   AND    INSECTS.  131 

flowers.     In  these  flowers  self-pollination  is  a  necessity,  and 
is  found  to  be  very  effective  in  producing  seed. 

91.  Yucca  and  Pronuba. — There  can  he  no  doubt,  also, 
that  there  is  a  great  deal  of  self-pollination  effected  in 
flowers  adapted  for  pollination  by  insects,  and  that  the  in- 
sects themselves  are  often  responsible  for  it.  But  in  the 
remarkable  case  of  Yucca  and  Pronuba  there  is  a  definite 
arrangement  for  self-pollination  by  means  of  an  insect  (see 
Fig.  130).  Yucca  is  a  plant  of  the  southwestern  arid  regions 
of  North  America,  and  Pronuba  is  a  moth.  The  plant  and 
the  moth  are  very  dependent  upon  each  other.  The  bell- 
shaped  flowers  of  Yucca  hang  in  great  terminal  clusters,  with 
six  hanging  stamens,  and  a  central  ovary  ribbed  lengthwise, 
and  with  a  funnel-shaped  opening  at  its  apex,  which  is  the 
stigma.  The  numerous  ovules  occur  in  lines  beneath  the 
furrows.  During  the  day  the  small  female  Pronuba  rests 
quietly  within  the  flower,  but  at  dusk  becomes  very  active. 
She  travels  down  the  stamens,  and  resting  on  the  open 
pollen-sac  scoops  out  the  somewhat  sticky  pollen  with  her 
front  legs.  Holding  the  little  mass  of  pollen  she  runs  to 
the  ovary,  stands  astride  one  of  the  furrows,  and  pierc- 
ing through  the  wall  with  her  ovipositor,  deposits  an  egg 
in  an  ovule.  After  depositing  several  eggs  she  runs  to  the 
apex  of  the  ovary  and  begins  to  crowd  the  mass  of  pollen 
she  has  collected  into  the  funnel-like  stigma.  These  actions 
are  repeated  several  times,  until  many  eggs  are  deposited 
and  repeated  pollination  has  been  effected.  As  a  result  of 
all  this  the  flower  is  pollinated,  and  seeds  are  formed  which 
develop  abundant  nourishment  for  the  moth  larvae,  which 
become  mature  and  bore  their  way  out  through  the  wall  of 
the  capsule  (Fig.  136). 

92.  Securing  cross-pollination. — In  very  many  ways  flow- 
ers are  adapted  to  the  visits  of  suitable  insects.  In  ob- 
taining nectar  or  pollen  as  food,  the  visiting  insect  receives 
pollen  on  some  part  of  its  body  which  will  be  likely  to 
come  in  contact  with  the  stigma  of  the  next  flower  visited. 


1 

L 

r 

]/ 

, 

rm^m 

w 


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w 


X' 


m 


Fig.  137.     A  clump  of  lady-slippers  (Cypripediam),  showing  the  habit  of  the  plant 
and  the  general  structure  of  the  flower.— After  Gibson. 


FLOWERS    AND    INSECTS. 


133 


Illustrations  of  this  process  may  be  taken  from  the  flowers 
already  described  in  connection  with  the  prevention  of 
self-pollination. 

In  the  flowers  of  the  pea  family,  such  as  the  rose  acacia 
(see  Fig.  131),  it  will  be 
noticed  that  the  stamens 
and  pistil  are  concealed 
within  the  keel,   which 
forms  the  natural  land- 
ing  place   for   the  bees 
which  are  used  in  pol- 
lination.     This   keel   is 
so   inserted    that   the 
weight  of  the  insect  de- 
presses it,   and   the  tip 
of    the    style   comes   in 
contact   with   its    body. 
Not   only  does   the 
stigma  strike  the  body, 
but   by   the    glancing 
blow  the  surface  of  the 
style  is  rubbed   against 
the  insect,  and  on  this 
style,  below  the  stigma, 
the  pollen  has  been  de- 
posited   and    is   rubbed 
oif    against   the   insect. 
At    the    next    flower 
visited     the     stigma    is 
likely  to  strike  the  pol- 
len obtained  from  the  previous  flower,  and  the  style  will 
deposit  a  new  supply  of  pollen. 

In  the  flower  of  the  common  flag  (see  Fig.  132)  the  nectar 
is  deposited  in  a  pit  at  the  bottom  of  the  chamber  formed 
by  each  style  and  petal.  In  this  chamber  the  stamen  is 
found,  and  more  or  less  roofing  it  over  is  the  flap,  or  shelf. 


Fig.  138.  Flower  of  Cypripedium,  showing  the 
flap  overhanging  the  opening  of  the  pouch, 
into  which  a  bee  is  crowding  its  way.  The 
small  figure  to  the  right  shows  a  side  view  (if 
the  flap ;  that  to  the  left  a  view  beneath  the 
flap,  showing  the  two  dark  anthers,  and  be- 
tween them,  further  down  (forward),  the 
stigma  surface.— After  Gibson. 


134 


PLANT    RELATIONS. 


upon  the  upper  surface  of  which  the  stigma  is  developed. 
As  the  insect  crowds  its  way  into  this  narrowing  chamber, 
its  body  is  dusted  by  the  pollen,  and  as  it  visits  the  next 
flower  and  thrusts  aside  the  stigmatic  shelf,  it  is  apt  to 
deposit  upon  it  some  of  the  pollen  previously  received. 

The  story  of  pollination  in  connection  with  the  orchids 
is  still  more  complicated  (see  Fig.  133).  Taking  an  ordi- 
nary orchid  for  illustration,  the  details  are  as  follows.  Each 
of  the  two  pollen  masses  terminates  in  a  sticky  disk  or 
button  ;  between  them  extends  the  concave  stigma  sur- 
face, at  the  bottom  of  which  is  the  opening  into  the  long 
tube-like  spur  in  which  the  nectar  is 
found.  Such  a  flower  is  adapted  to 
the  large  moths,  with  long  probosces 
which  can  reach  the  bottom  of  the 
tube.  As  the  moth  thrusts  its  pro- 
boscis into  the  tube,  its  head  touches 
the  sticky  button  on  each  side,  so  that 
when  it  flies  away  these  buttons  stick 
to  its  head,  sometimes  directly  to  its 
eyes,  and  the  pollen  masses  are  torn 
out.  These  masses  are  then  carried 
to  the  next  flower  and  are  thrust 
against  the  stigma  in  the  attempt  to  get  the  nectar. 

In  the  lady-slipper  (Cypripedium),  another  orchid,  the 
flowers  have  a  conspicuous  pouch  (see  Fig.  137),  in  which 
the  nectar  is  secreted.  A  peculiar  structure,  like  a  flap, 
overhangs  the  opening  of  the  pouch,  beneath  which  are  the 
two  anthers,  and  between  them  the  stigmatic  surface  (see 
Fig.  138).  Into  the  pouch  a  bee  crowds  its  way  and  be- 
comes imprisoned  (see  Fig.  139).  The  nectar  which  the 
bee  obtains  is  in  the  bottom  of  the  pouch  (see  Fig.  140). 
When  escaping,  the  bee  moves  towards  the  opening  over- 
hung by  the  flap  and  rubs  first  against  the  stigmatic  sur- 
face (see  Fig.  141),  and  then  against  the  anthers,  receiving 
pollen  on  its  back  (see  Fig.  142).     A  visit  to  another  flower 


Fig.  139.  A  bee  imprisoned 
in  the  pouch  (partly  cut 
away)  of  Cypripediwn. 
—After  Gibson. 


FLOWERS   AND   INSECTS. 


135 


A  bee  obtaining  nectar  in  the   pouch  of 
CypHpedium.  — After  Gibson. 


will  result  in  rubbing  some  of  the  pollen  upon  the  stigma, 
and  in  receiving  more  pollen  for  another  flower. 

In  cases  of  protandry,  as  the  common  figwort,  flowers 
in  the   two  condi- 
tions will  be  visited 
by  the  pollinating 
insect,  and  as  the 
shedding    stamens 
and  receptive  stig- 
mas   occupy    the 
same  relative  posi- 
tion,   the    pollen 
from    one    flower 
will  be  carried  to  the  stigma  of  another.     It  is  evident  that 
exactly  the  same  methods  prevail  in  the  case  of  protogyny, 
as  the  fire  weed  (see  Fig.  134). 

The  Iloustonia  (see  Fig.  135),  in  which  there  are  sta- 
mens and  styles  of  different  lengths,  is  visited  by  insects 

whose  bodies  fill 
the  tube  and  pro- 
trude above  it.  In 
visiting  flowers  of 
both  kinds,  one  re- 
gion of  the  body 
receives  pollen 
from  the  short  sta- 
mens, and  another 
region  from  the 
long  stamens.  In 
this  way  the  insect 
which  come  in  con- 
When  there  are  three 


Fig.  141.  A  bee  escaping  from  the  pouch  of  Cypri- 
pedium, and  coming  in  contact  with  the  stigma. 
Advancing  a  little  further  the  bee  will  come  in  con- 
tact with  the  anthers  and  receive  pollen.— After 
Gibson. 


will  carry  about  two  bands  of  pollen 

tact  with  the  corresponding  stigmas. 

forms  of  flowers,  as  mentioned  in  the  case  of  one  of  the 

loosestrifes,  the  insect  receives  three  pollen  bands,  one  for 

each  of  the  three  sets  of  stigmas. 

93.  Warding  off  unsuitable  insects. — Prominent   among 
10 


136 


PLANT    RELATIONS. 


the  unsuitable  insects,  which  Kerner  calls  "unbidden 
guests/''  are  ants,  and  adaptations  for  reducing  their  visits 
to  a  minimum  may  be  taken  as  illustrations. 

(1)  Hairs. — A  common  device  for  turning  back  ants, 
and  other  creeping  insects,  is  a  barrier  of  hair  on  the  stem, 
or  in  the  flower  cluster,  or  in  the  flower. 

(2)  Glandular  secretions. — In  some  cases  a  sticky 
secretion   is   exuded   from   the   surface   of   plants,    which 

effectively  stops 
the  smaller  creep- 
ing insects.  In 
certain  species  of 
catch-fly  a  sticky 
ring  girdles  each 
joint  of  the  stem. 

(3)  Isolation. — 
The  leaves  of  cer- 
tain plants  form 
water  reservoirs 
about  the  stem. 
To  ascend  such  a 
stem,  therefore,  a 
creeping  insect 
must  cross  a  series 
of  such  reservoirs. 
Teasel  furnishes  a 
common  illustration,  the  opposite  leaves  being  united  at 
the  base  and  forming  a  series  of  cups.  More  extensive 
water  reservoirs  are  found  in  Bilbergia,  sometimes  called 
"  traveler's  tree,"  whose  great  flower  clusters  are  pro- 
tected by  large  reservoirs  formed  by  the  rosettes  of  leaves, 
which  creeping  insects  cannot  cross. 

(4)  Latex. — This  is  a  milky  secretion  found  in  some 
plants,  as  in  milkweeds.  Caoutchouc  is  a  latex  secretion 
of  certain  tropical  trees.  When  latex  is  exposed  to  the 
air   it   stiffens   immediately,   becoming  sticky  and  finally 


Fig.  142.  A  bee  escaping  from  the  pouch  of  Cypri- 
pedium,  and  rubbing  against  an  anther.— After 
Gibson. 


FLOWERS    AND    INSECTS.  137 

hard.  In  the  flower  clusters  of  many  latex-secreting 
plants  the  epidermis  of  the  stem  is  very  smooth  and  deli- 
cate, and  easily  pierced  by  the  claws  of  ants  and  other 
creeping  insects  who  seek  to  maintain  footing  on  the 
smooth  surface.  Wherever  the  epidermis  is  pierced  the 
latex  gushes  out,  and  by  its  stiffening  and  hardening  glues 
the  insect  fast. 

(5)  Protective  forms. — In  some  cases  the  structure  of 
the  flower  prevents  the  access  of  small  creeping  insects  to 
the  pollen  or  to  the  nectar.  In  the  common  snapdragon 
the  two  lips  are  firmly  closed  (see  Fig.  7-4),  and  they  can  be 
forced  apart  only  by  some  heavy  insect,  as  the  bumble-bee, 
alighting  upon  the  projecting  lower  lip,  all  lighter  insects 
being  excluded.  In  many  species  of  Pentstemon,  one  of 
the  stamens  does  not  develop  pollen  sacs,  but  lies  like  a  bar 
across  the  mouth  of  the  pit  in  which  the  nectar  is  secreted. 
Through  the  crevices  left  by  this  bar  the  thin  proboscis  of 
a  moth  or  butterfly  can  pass,  but  not  the  whole  body  of  a 
creeping  insect.  Very  numerous  adaptations  of  this  kind 
may  be  observed  in  different  flowers. 

(6)  Protective  closure.—  Certain  flowers  are  closed  at 
certain  hours  of  the  day,  when  there  is  the  chief  danger 
from  creeping  insects.  For  instance,  the  evening  prim- 
roses open  at  dusk,  after  the  deposit  of  dew,  when  ants  are 
not  abroad  ;  and  at  the  same  time  they  secure  the  visits  of 
moths,  which  are  night-fliers. 

Numerous  other  adaptations  to  hinder  the  visits  of 
unsuitable  insects  may  be  observed,  but  those  given  will 
serve  as  illustrations. 


CHAPTER  VIII. 

AN   INDIVIDUAL   PLANT   IN   ALL   OF   ITS   RELATIONS. 

For  the  purpose  of  summarizing  the  general  life-rela- 
tions detailed  in  the  preceding  chapters,  it  will  be  useful  to 
apply  them  in  the  case  of  a  single  plant.  Taking  a  com- 
mon seed-plant  as  an  illustration,  and  following  its  history 
from  the  germination  of  the  seed,  certain  general  facts 
become  evident  in  its  relations  to  the  external  world. 

94.  Germination  of  the  seed. — The  most  obvious  needs  of 
the  seed  for  germination  are  certain  amounts  of  moisture 
and  heat.  In  order  to  secure  these  to  the  best  advantage, 
the  seed  is  usually  very  definitely  related  to  the  soil,  either 
upon  it  and  covered  by  moisture  and  heat-retaining  debris. 
or  embedded  in  it.  Along  with  the  demand  for  heat  and 
moisture  is  one  for  air  (supplying  oxygen),  which  is  essen- 
tial to  life.  The  relation  which  germinating  seeds  need, 
therefore,  is  one  which  not  only  secures  moisture 'and  heat 
advantageously,  but  permits  a  free  circulation  of  air. 

95.  Direction  of  the  root. — The  first  part  of  the  young 
plantlet  to  emerge  from  the  seed  is  the  tip  of  the  axis 
which  is  to  develop  the  root  system.  It  at  once  appears  to 
be  very  sensitive  to  the  earth  influence  {gcotropism)  and 
to  moisture  influence  (hydrotropism),  for  whatever  the 
direction  of  emergence  from  the  seed,  a  curvature  is  devel- 
oped which  directs  the  tip  towards  and  finally  into  the  soil 
(see  Fig.  143).  When  the  soil  is  penetrated  the  primary 
root  may  continue  to  grow  vigorously  downward,  showing 
a  strong  geotropic  tendency,  and  forming  what  is  known 
as  the  tap-root,  from  which  lateral  roots  arise,  which  are 


AN    INDIVIDUAL   PLANT   IN   ALL   OF   ITS   RELATIONS.     189 


much  more  influenced  in  direction  by  other  external 
causes,  especially  the  presence  of  moisture.  As  a  rule, 
the  soil  is  not  perfectly  uniform,  and  contact  with  different 
substances  induces  curvatures,  and  as  a  result  of  these  and 
other  causes,  the  root  system  may  become  very  intricate, 
which  is  extremely  favor- 
able for  absorbing  and 
gripping. 

9G.  Direction  of  the  stem. 
— As  soon  as  the  stem  tip 
is  extricated  from  the  seed, 
it  exhibits  sensitiveness  to 
the  light  influence  (heliot- 
ropism),  being  guided  in 
a  general  way  towards  the 
light  (see  Fig.  143«). 
Direction  towards  the 
light,  the  source  of  the  in- 
fluence, is  spoken  of  as 
positive  heliotropism,  as 
from    direc- 


h 


B 


distinguished 


tion    away 


from  the  light, 


Fig,  143.  Germination  of  the  seed  of 
arbor-vitae  (Thuja).  B  shows  the 
emergence  of  the  axis  (/)  which  is  to 
develop  the  root,  and  its  turning  to- 
wards the  soil.  C  shows  a  later  stage, 
in  which  the  root  (r)  has  been  some- 
what developed,  and  the  stem  of  the 
embryo  (h)  is  developing  a  curve  pre- 
paratory to  pulling  out  the  seed  leaves 
(cotyledons).  E  shows  the  young  plant- 
let  entirely  free  from  the  seed,  with  its 
root  (r)  extending  into  the  soil,  its  stem 
(h)  erect,  and  its  fust  leaves  (c)  hori- 
zontally spread.— After  Strasburger. 


called  negative  heliotro- 
pism. If  the  main  axis 
continues  to  develop,  it 
continues  to  show  this  posi- 
tive heliotropism.  strongly, 
but  the  branches  may  show 
every  variation  from  positive  to  transverse  heliotropism  ; 
that  is,  a  direction  transverse  to  the  direction  of  the  rays 
of  light.  In  some  plants  certain  stems,  as  stolons,  run- 
ners, etc.,  show  strong  transverse  heliotropism,  while  other 
stems,  as  rootstocks,  etc.,  show  a  strong  transverse  geot- 
ropism. 

97.  Direction  of  foliage  leaves. — The  general  direction  of 
foliage  leaves  on  an  erect  stem  is  transversely  heliotropic  ; 


140 


PLANT   RELATIONS. 


if  necessary,  the  parts  of  the  leaf  or  the  stem  itself  twisting 
to  allow  the  blade  to  assume  this  position.  The  danger  of 
the  leaves  shading  one  another  is  reduced  to  a  minimum  by 
the  elongation  of  internodes,  the  spiral  arrangement,  short- 
ening and  changing  direction  upwards,  or  lobing. 

This  outlines  the  general  nutritive  relations,  the  roots 


Fig.  143«.  Germination  of  the  garden  bean,  showing  the  arch  of  the  seedling  stem 
above  ground,  its  pull  on  the  seed  to  extricate  the  cotyledons  and  plumule,  and 
the  final  straightening  of  the  stem  and  expansion  of  the  young  leaves.— After 
Atkinson. 


and  leaves  being  favorably  placed  for  absorption,  and  the 
latter  also  favorably  placed  for  photosynthesis. 

98.  Placing  of  flowers. — The  purposes  of  the  flower  seem 
to  be  served  best  by  exposed  positions,  and  consequently 
flowers  mostly  appear  at  the  extremities  of  stems  and 
branches,  a  position  evidently  favorable  to  pollination  and 
seed  dispersal.  The  flowers  thus  exposed  are  very  com- 
monly massed,  or,  if  not,  the  single  flower  is  apt  to  be  large 
and  conspicuous.  The  various  devices  for  protecting  nec- 
tar and  pollen  against  too  great  moisture,  and  the  more 


AX   INDIVIDUAL   PLANT   IN   ALL   OF  ITS   RELATIONS.    14i 

delicate  structures  against  chill ;  for  securing  the  visits  of 
suitable  insects,  and  warding  off  unsuitable  insects;  and 
for  dispersing  the  seeds,  need  not  be  repeated. 

99.  Branch  buds. — If  the  plant  under  examination  be  a 
tree  or  shrub,  branch  buds  will  be  observed  to  be  developed 
during  the  growing  season  (see  Fig.  05).  This  device 
for  protecting  growing  tips  through  a  season  of  dangerous 
cold  is  very  familiar  to  those  living  in  the  temperate 
regions.  The  internodes  do  not  elongate,  hence  the  leaves 
overlap  ;  they  develop  little  or  no  chlorophyll,  and  become 
scales.  The  protection  afforded  by  these  overlapping 
scales  is  often  increased  by  the  development  of  hairs,  or 
by  the  secretion  of  mucilage  or  gum. 


CHAPTER   IX. 

THE   STRUGGLE   FOR  EXISTENCE. 

100.  Definition. — The  phrase  "struggle  for  existence" 
has  come  to  mean,  so  far  as  plants  are  concerned,  that  it  is 
usually  impossible  for  them  to  secure  ideal  relations,  and 
that  they  must  encounter  unfavorable  conditions.  The 
proper  light  and  heat  relations  may  be  difficult  to  obtain, 
and  also  the  proper  relations  to  food  material.  It  often 
happens,  also,  that  conditions  once  fairly  favorable  may  be- 
come unfavorable.  Also,  multitudes  of  plants  are  trying 
to  take  possession  of  the  same  conditions.  All  this  leads 
to  the  so-called  "struggle/'  and  vastly  more  plants  fail 
than  succeed.  Before  considering  the  organization  of  plant 
societies,  it  will  be  helpful  to  consider  some  of  the  possible 
changes  in  conditions,  and  the  effect  on  plants. 

101.  Decrease  of  water. — This  is  probably  the  most  com- 
mon factor  to  fluctuate  in  the  environment  of  a  plant. 
Along  the  borders  of  streams  and  ponds,  and  in  swampy 
places,  the  variation  in  the  water  is  very  noticeable,  but  the 
same  thing  is  true  of  soils  in  general.  However,  the  change 
chiefly  referred  to  is  that  which  is  permanent,  and  which 
compels  plants  not  merely  to  tide  over  a  drought,  but  to 
face  a  permanent  decrease  in  the  water  supply. 

Around  the  margins  of  ponds  are  very  commonly  seen 
fringes  of  such  plants  as  bulrushes,  cat- tail  flags,  reed- 
grasses,  etc.,  standing  in  shoal  water.  As  these  plants 
grow  close  together,  silt  from  the  land  is  entangled  by  them, 
and  presently  it  accumulates  to  such  an  extent  that  there 
is  no  more  standing  water,  and  the  water  supply  for  the 


THE    STRUGGLE    FOR   EXISTENCE.  143 

bulrushes  and  their  associates  has  permanently  decreased 
below  the  favorable  amount.  In  this  way  certain  lake 
margins  gradually  encroach  upon  the  water,  and  in  so 
doing  the  water  supply  is  permanently  diminished  for  many 
plants.  By  the  same  process,  smaller  lakelets  are  gradually 
being  converted  into  bogs,  and  the  bogs  in  turn  into  drier 
ground,  and  these  unfavorable  changes  in  water  supply  are 
a  menace  to  many  plants. 

The  operations  of  man,  also,  have  been  very  effective  in 
diminishing  the  water  supply  for  plants.  Drainage,  which 
is  so  extensively  practiced,  while  it  may  make  the  water- 
supply  more  favorable  for  the  plants  which  man  desires,  cer- 
tainly makes  it  very  unfavorable  for  many  other  plants. 
The  clearing  of  forests  has  a  similar  result.  The  forest 
soil  is  receptive  and  retentive  in  reference  to  water,  and  is 
somewhat  like  a  great  sponge,  steadily  supplying  the  streams 
which  drain  it.  The  removal  of  the  forest  destroys  much 
of  this  power.  The  water  is  not  held  and  gradually  doled 
out,  but  rushes  off  in  a  flood  ;  hence,  the  streams  which 
drain  the  cleared  area  are  alternately  flooded  and  dried  up. 
This  results  in  a  much  less  total  supply  of  water  available 
for  the  use  of  plants. 

102.  Decrease  of  light. — It  is  very  common  to  observe 
tall,  rank  vegetation  shading  lower  forms,  and  seriously 
interfering  with  the  light  supply.  If  the  rank  vegetation 
is  rather  temporary,  the  low  plants  may  learn  to  precede  or 
follow  it,  and  so  avoid  the  shading  ;  but  if  the  over-shading 
vegetation  is  a  forest  growth,  shading  becomes  permanent. 
In  the  case  of  deciduous  trees,  which  drop  their  leaves  at  the 
close  of  the  growing  season  and  put  out  a  fresh  crop  in  the 
spring,  there  is  an  interval  in  the  early  spring,  before  the 
leaves  are  fully  developed,  during  which  low  plants  may 
secure  a  good  exposure  to  light  (see  Fig.  144).  In  such 
places  one  finds  an  abundance  of  "spring  flowers."  but  later 
in  the  season  the  low  plants  become  very  scarce.  This 
effective  over-shading  is  not   common   to    all   forests,  for 


Fig.  144.  A  common  spring  plant  (dog-tooth  violet)  which  grows  in  deciduous 
forests.  The  large  mottled  leaves  and  the  conspicuous  flowers  are  sent  rapidly 
above  the  surface  from  the  subterranean  bulb  (see  cut  in  the  left  lower  corner), 
where  are  also  seen  dissected  out  some  petals  and  stamens  and  the  pistil. 


THE   STRUGGLE   FOR   EXISTENCE.  145 

there  are  "light  forests/'  such  as  the  oak  forest,  which 
permit  much  low  vegetation,  as  well  as  the  shade  forests, 
such  as  beech  forests,  which  permit  very  little. 

In  the  forest  regions  of  the  tropics,  however,  the  shad- 
ing is  permanent,  since  there  is  no  annual  fall  of  leaves. 
In  such  conditions  the  climbing  habit  has  been  extensively 
cultivated. 

103.  Change  in  temperature.— In  regions  outside  of  the 
tropics  the  annual  change  of  temperature  is  a  very  im- 
portant factor  in  the  life  of  plants,  and  they  have  provided 
for  it  in  one  way  or  another.  In  tracing  the  history  of 
plants,  however,  back  into  what  are  called  "geological 
times,"  we  discover  that  there  have  been  relatively  per- 
manent changes  in  temperature.  Now  and  then  glacial 
conditions  prevailed,  during  which  regions  before  temperate 
or  even  tropical  were  subjected  to  arctic  conditions.  It  is 
very  evident  that  such  permanent  changes  of  temperature 
must  have  had  an  immense  influence  upon  plant  life. 

104.  Change  in  soil  composition. — One  of  the  most  ex- 
tensive agencies  in  changing  the  compositions  of  soils  in 
certain  regions  has  been  the  movement  of  glaciers  of  conti- 
nental extent,  which  have  deposited  soil  material  over  very 
extensive  areas.  Areas  within  reach  of  occasional  floods, 
also,  may  have  the  soil  much  changed  in  character  by  the 
new  deposits.  Shifting  dunes  are  billow-like  masses  of 
sand,  developed  and  kept  in  motion  by  strong  prevailing 
winds,  and  often  encroach  upon  other  areas.  Besides  these 
changes  in  the  character  of  soil  by  natural  agencies,  the 
various  operations  of  man  have  been  influential.  Clearing, 
draining,  fertilizing,  all  change  the  character  of  the  soil, 
both  in  its  chemical  composition  and  its  physical  properties. 

105.  Devastating  animals. — The  ravages  of  animals  form 
an  important  factor  in  the  life  of  many  plants.  For  example, 
grazing  animals  are  wholesale  destroyers  of  vegetation,  and 
may  seriously  affect  the  plant  life  of  an  area.  The  various 
leaf  feeders  among  insects   have   frequently  done  a  vast 


146  PLANT   RELATIONS. 

amount  of  damage  to  plants.  Many  burrowing  animals 
attack  subterranean  parts  of  plants,  and  interfere  seriously 
with  their  occupation  of  an  area. 

Various  protective  adaptations  against  such  attacks  have 
been  pointed  out,  but  this  subject  probably  has  been  much 
exaggerated.  The  occurrence  of  hairs,  prickles,  thorns, 
and  spiny  growths  upon  many  plants  may  discourage  the 
attacks  of  animals,  but  it  would  be  rash  to  assume  that 
these  protections  have  been  developed  because  of  the  danger 
of  such  attacks.  One  of  the  families  of  plants  most  com- 
pletely protected  in  this  way  is  the  great  cactus  family, 
chiefly  inhabiting  the  arid  regions  of  southwestern  United 
States  and  Mexico.  In  such  a  region  succulent  vegetation 
is  at  a  premium,  and  it  is  doubtless  true  that  the  armor  of 
thorns  and  bristles  reduces  the  amount  of  destruction. 

In  addition  to  armor,  the  acrid  or  bitter  secretions  of 
certain  plants  or  certain  parts  of  plants  would  have  a 
tendency  to  ward  oil  the  attacks  of  animals. 

106.  Plant  rivalry. — It  is  evident  that  there  must  be 
rivalry  among  plants  in  occupying  an  area,  and  that  those 
plants  which  can  most  nearly  utilize  identical  conditions 
will  be  the  most  intense  rivals.  For  example,  a  great  many 
young  oaks  may  start  up  over  an  area,  and  it  is  evident 
that  the  individuals  must  come  into  sharp  competition  with 
one  another,  and  that  but  few  of  them  succeed  in  establish- 
ing themselves  permanently.  This  is  rivalry  between  in- 
dividuals of  the  same  kind  ;  but  some  other  kind  of  trees, 
as  the  beech,  may  come  into  competition  with  the  oak,  and 
another  form  of  rivalry  will  appear. 

As  a  consequence  of  plant  rivalry,  the  different  plants 
which  finally  succeed  in  taking  possession  of  an  area  are 
apt  to  be  dissimilar,  and  a  plant  society  is  usually  made  up 
of  plants  which  represent  widely  different  regions  of  the 
plant  kingdom.  It  is  sometimes  said  that  any  well  de- 
veloped plant  society  is  an  epitome  of  the  plant  kingdom. 

A  familiar  illustration  of  plant  rivalry  may  be  observed 


THE    STRUGGLE    FOR   EXISTENCE.  147 

in  the  case  of  what  are  called  "weeds."  Every  one  is  fa- 
miliar with  the  fact  that  if  cultivated  ground  is  neglected 
these  undesirable  plants  will  invade  it  vigorously  and  seri- 
ously affect  the  development  of  plants  under  cultivation. 

107.  Adaptation. — When  the  changes  mentioned  above 
occur  in  the  environment  of  plants  to  such  an  extent  as 
to  make  the  conditions  for  living  very  unfavorable,  one 
of  three  things  is  likely  to  occur,  adaptation,  migration, 
or  destruction. 

The  change  in  conditions  may  come  slowly  enough,  and 
certain  plants  may  be  able  to  endure  it  long  enough  to 
adjust  themselves  to  it.  Such  an  adjustment  may  involve 
changes  in  structure,  and  probably  no  plants  are  plastic 
enough  to  adjust  themselves  to  extreme  and  sudden  changes 
which  are  to  be  comparatively  permanent.  There  are 
plants,  such  as  the  common  cress,  which  may  be  called 
amphibious,  which  can  live  in  the  water  or  out  of  it  without 
change  of  structure,  but  this  is  endurance  rather  than 
adaptation.  Many  plants,  however,  can  pass  slowly  into 
different  conditions,  such  as  drier  soil,  denser  shade,  etc., 
and  corresponding  changes  in  their  structure  may  be  noted. 
Very  often,  however,  such  plants  are  given  no  opportunity 
to  adjust  themselves  to  the  new  conditions,  as  the  area  is 
apt  to  be  invaded  by  plants  already  better  adapted.  While 
adaptation  may  be  regarded  as  a  real  result  of  changed  con- 
ditions, it  would  seem  to  be  by  no  means  the  common  one. 
108.  Migration. — This  is  a  very  common  result  of 
changed  conditions.  Plants  migrate  as  truly  as  animals, 
though,  of  course,  their  migration  is  from  generation  to 
generation.  It  is  evident,  however,  that  migration  cannot 
be  universal,  for  barriers  of  various  kinds  may  forbid  it. 
In  general,  these  barriers  represent  unfavorable  conditions 
for  living.  If  a  plant  area  with  good  soil  is  surrounded  by 
a  sterile  area,  the  latter  would  form  an  efficient  barrier  to 
migration  from  the  former.  Plants  of  the  lowlands  could 
not  cross  mountains  to  escape  from  unfavorable  conditions. 


148  PLANT   RELATIONS. 

To  make  migration  possible,  therefore,  it  is  necessary  for 
the  conditions  to  be  favorable  for  the  migrating  plants  in 
some  direction.  In  the  case  of  bulrushes,  cat-tail  flags, 
etc.,  growing  in  the  shoal  water  of  a  lake  margin,  the 
building  up  of  soil  about  them  results  in  unfavorable  con- 
ditions. As  a  consequence,  they  migrate  further  into  the 
lake.  If  the  lake  happens  to  be  a  small  one,  the  filling  up 
process  may  finally  obliterate  it,  and  a  time  will  come  when 
such  forms  as  bulrushes  and  flags  will  find  it  impossible  to 
migrate. 

In  glacial  times  very  many  arctic  plants  migrated  south- 
ward, especially  along  the  mountain  systems,  and  many 
alpine  plants  moved  to  lower  ground.  When  warmer  con- 
ditions returned,  many  plants  that  had  been  driven  south 
returned  towards  the  north,  and  the  arctic  and  alpine  plants 
retreated  to  the  north  and  up  the  mountains.  The  history 
of  plants  is  full  of  migrations,  compelled  by  changed  con- 
ditions and  permitted  in  various  directions.  It  must  be 
remembered,  also,  that  migrations  often  result  in  changes 
of  structure. 

109.  Destruction. — Probably  this  is  by  far  the  most  com- 
mon result  of  greatly  changed  conditions.  Even  if  plants 
adapt  themselves  to  changed  conditions,  or  migrate,  their 
structure  may  be  so  changed  that  they  will  seem  like  quite 
different  plants.  In  this  way  old  forms  gradually  disappear 
and  new  ones  take  their  places. 


CHAPTER  X. 

THE    NUTRITION    OF    PLANTS. 

110.  Physiology. — In  the  previous  chapters  plants  have 
been  considered  in  reference  to  their  surroundings.  It 
was  observed  that  various  organs  of  nutrition  hold  certain 
life-relations,  but  it  is  essential  to  discover  what  these  rela- 
tions mean  to  the  life  of  the  plant.  The  study  of  plants 
from  the  standpoint  of  their  life-relations  has  been  called 
Ecology  ;  the  study  of  the  life-processes  of  plants  is  called 
Physiology.  These  two  points  of  view  may  be  illustrated 
by  comparing  them  to  two  points  of  view  for  the  study  of 
man.  Man  may  be  studied  in  reference  to  his  relation  to 
his  fellow-men  and  to  the  character  of  the  country  in  which 
he  lives  ;  or  his  bodily  processes  may  be  studied,  such  as 
digestion,  circulation,  respiration,  etc.  The  former  cor- 
responds to  Ecology,  the  latter  is  Physiology. 

All  of  the  ecological  relations  that  have  been  mentioned 
find  their  meaning  in  the  physiology  of  the  plant,  for  life- 
relations  have  in  view  life-processes.  The  subject  of  plant 
physiology  is  a  very  complex  one,  and  it  would  be  impossi- 
ble in  an  elementary  work  to  present  more  than  a  few  very 
general  facts.  Certain  facts  in  reference  to  plant  move- 
ments, an  important  physiological  subject,  have  been  men- 
tioned in  connection  with  life-relations,  but  it  seems  neces- 
sary to  make  some  special  mention  of  nutrition. 

111.  Significance  of  chlorophyll. — Probably  the  most  im- 
portant fact  to  observe  in  reference  to  the  nutrition  of 
plants  is  that  some  plants  are  green  or  have  green  parts, 
while  others,  such  as  toadstools,  do  not  show  this  green 


150  PLANT   RELATIONS. 

color.  It  has  been  stated  that  this  green  color  is  due  to 
the  presence  of  a  coloring  matter  known  as  chlorophyll 
(see  §12).  The  two  groups  may  be  spoken  of,  therefore, 
as  (1)  green  plants  and  (2)  plants  ivithout  chlorophyll. 
The  presence  of  chlorophyll  makes  it  possible  for  the  plants 
containing  it  to  manufacture  their  own  food  out  of  such 
materials  as  water,  soil  material,  and  gases.  For  this 
reason,  green  plants  may  be  entirely  independent  of  all 
other  living  things,  so  far  as  their  food  supply  is  concerned. 

Plants  without  chlorophyll,  however,  are  unable  to 
manufacture  food  out  of  such  materials,  and  must  obtain 
it  already  manufactured  in  the  bodies  of  other  plants  or 
animals.  For  this  reason,  they  are  dependent  upon  other 
living  things  for  their  food  supply,  just  as  are  animals.  It 
is  evident  that  plants  without  chlorophyll  may  obtain  this 
food  simply  either  from  the  living  bodies  of  plants  and  ani- 
mals, in  which  case  they  are  called  parasites,  or  they  may 
obtain  it  from  the  substances  derived  from  the  bodies  of 
plants  and  animals,  in  which  case  they  are  called  sapro- 
phytes. For  example,  the  rust  which  attacks  the  wheat, 
and  is  found  upon  the  leaves  and  stems  of  the  living  plant, 
is  a  parasite ;  while  the  mould  which  often  develops  on  stale 
bread  is  a  saprophyte.  Some  plants  without  chlorophyll 
can  live  either  as  parasites  or  saprophytes,  while  others  are 
always  one  or  the  other.  By  far  the  largest  number  of 
parasites  and  saprophytes  belong  to  the  group  of  low  plants 
called  fungi,  and  when  fungi  are  referred  to,  it  must  be 
understood  that  it  means  the  greatest  group  of  plants  with- 
out chlorophyll. 

112.  Photosynthesis. — The  nutritive  processes  in  green 
plants  are  the  same  as  in  other  plants,  and  in  addition  there 
is  in  green  plants  the  peculiar  process  known  as  photosyn- 
thesis (see  §25).  In  plants  with  foliage  leaves,  these  are 
the  chief  organs  for  this  work.  It  must  be  remembered, 
however,  that  leaves  are  not  necessary  for  photosynthesis, 
for  plants  without  leaves,  such  as  alga?,  perform  it.       The 


THE   NUTRITION   OF   PLANTS.  151 

essential  thing  is  green  tissue  exposed  to  light,  but  in  this 
brief  account  an  ordinary  leafy  plant  growing  in  the  soil 
will  be  considered. 

As  the  leaves  are  the  active  structures  in  the  work  of 
photosynthesis,  the  raw  materials  necessary  must  be  brought 
to  them.  In  a  general  way,  these  materials  are  carbon  di- 
oxide and  water.  The  gas  exists  diffused  through  the 
atmosphere,  and  so  is  in  contact  with  the  leaves.  It  also 
occurs  dissolved  in  the  water  of  the  soil,  but  the  gas  used 
is  absorbed  from  the  air  by  the  leaves.  The  supply  of 
water,  on  the  other  hand,  in  soil-related  plants,  is  obtained 
from  the  soil.  The  root  system  absorbs  this  water,  which 
then  ascends  the  stem  and  is  distributed  to  the  leaves. 

(1)  Ascent  of  water. — The  water  does  not  move  up- 
wards through  all  parts  of  the  stem,  but  is  restricted  to  a 
certain  definite  region.  This  region  is  easily  recognized  as 
the  woody  part  of  stems.  Sometimes  separate  strands  of 
wood,  looking  like  fibers,  may  be  seen  running  lengthwise 
through  the  stem  ;  sometimes  the  fibrous  strands  are  packed 
so  close  together  that  they  form  a  compact  woody  mass,  as 
in  shrubs  and  trees.  In  the  case  of  most  trees  new  wood  is 
made  each  year,  through  which  the  water  moves.  Hence 
the  very  common  distinction  is  made  between  sap-wood, 
through  which  the  water  is  moving,  and  heart-wood,  which 
the  water  current  has  abandoned.  Just  how  the  water 
ascends  through  these  woody  fibers,  especially  in  tall  trees, 
is  a  matter  of  much  discussion,  and  cannot  be  regarded  as 
definitely  known.  In  any  event,  it  should  be  remembered 
that  these  woody  fibers  are  not  like  the  open  veins  and 
arteries  of  animal  bodies,  and  no  "  circulation  "  is  possible. 
These  same  woody  strands  are  seen  brandling  throughout 
the  leaves,  forming  the  so-called  vein  system,  and  it  is  evi- 
dent, therefore,  that  they  form  a  continuous  route  from 
roots  to  leaves. 

It  is  easy  to  demonstrate  the  ascent  of  water  in  the 
stem,  and  the  path  it  takes,  by  a  simple  experiment.  If 
11 


152  PLANT   RELATIONS. 

an  active  stem  be  cut  and  plunged  into  water  stained  with 
an  aniline  color  called  eosin,*  the  ascending  water  will  stain 
its  pathway.  After  some  time  sections  through  the  stem 
will  show  that  the  water  has  traveled  upwards  through  it, 
and  the  stain  will  point  out  the  region  of  the  stem  used  in 
the  movement. 

In  general,  therefore,  the  carbon  dioxide  is  absorbed 
directly  from  the  air  by  the  leaves,  and  the  water  is  ab- 
sorbed by  the  root  from  the  soil,  and  moves  upwards  through 
the  stem  into  the  leaves.  An  interesting  fact  about  these 
raw  materials  is  that  they  are  very  common  waste  products. 
They  are  waste  products  because  in  most  life-processes  they 
cannot  be  taken  to  pieces  and  used.     The  fact  that  they 

can  be  used  in  photosynthesis 
shows  that  it  is  a  very  re- 
markable life  process. 

(2)  Cliloroplasts. — Having 
obtained  some  knowledge  of 
the  raw  materials  used  in 
photosynthesis,    and    their 

Fig.  145.    Some  mesophyll  cells  from      A  J 

the  leaf  of  Fittonia,  showing  chloro-      SOUrceS,      it      IS     necessary      to 

PlaBts-  consider  the  plant  machinery 

arranged  for  the  work.  In  the  working  leaf  cells  it  is 
discovered  that  the  color  is  due  to  the  presence  of  very 
small  green  bodies,  known  as  chlorophyll  bodies  or  cliloro- 
plasts (see  Fig.  145).  These  consist  of  the  living  substance, 
known  as  protoplasm,  and  the  green  stain  called  chloro- 
phyll ;  therefore,  each  chloroplast  is  a  living  body  (plastid) 
stained  green.  It  is  in  these  cliloroplasts  that  the  work  of 
photosynthesis  is  done.  In  order  that  they  may  work  it 
is  necessary  for  them  to  obtain  a  supply  of  energy  from 
some  outside  source,  and  the  source  used  in  nature  is  sun- 
light. The  green  stain  (chlorophyll)  seems  to  be  used  in 
absorbing   the   necessary  energy  from  sunlight,   and   the 

*  The  commoner  grades  of  red  ink  are  usually  solutions  of  eosin. 


THE    NUTRITION   OF   PLANTS.  153 

plastid  uses  this  energy  in  the  work  of  photosynthesis.  It 
is  evident,  therefore,  that  photosynthesis  goes  on  only  in 
the  sunlight,  unci  is  suspended  entirely  at  night.  It  is 
found  that  any  intense  light  can  be  used  as  a  substitute 
for  sunlight,  and  plants  have  been  observed  to  carry  on 
the  work  of  photosynthesis  in  the  presence  of  electric 
light. 

(3)  Result  of  photosynthesis. — The  result  of  this  work 
can  be  stated  only  in  a  very  general  way.  Carbon  dioxide 
is  composed  of  Wo  elements,  carbon  and  oxygen,  in  the 
proportion  one  part  of  carbon  to  two  parts  of  oxygen. 
Water  is  also  composed  of  two  elements,  hydrogen  and  oxy- 
gen. In  photosynthesis  the  elements  composing  these  sub- 
stances are  separated  from  one  another,  and  recombined  in 
a  new  way.  In  the  process  a  certain  amount  of  oxygen  is 
liberated,  just  as  much  as  was  in  the  carbon  dioxide,  and  a 
new  substance  is  formed,  known  as  a  carbohydrate.  The 
oxygen  set  free  escapes  from  the  plant,  and  may  be  re- 
garded as  waste  product  in  the  process  of  photosynthesis. 
It  will  be  remembered  that  the  external  changes  in  this 
process  are  the  absorption  of  carbon  dioxide  and  the  giving 
off  of  oxygen  (see  §25). 

(4)  Carbohydrates  and  proteids. — The  carbohydrate 
formed  is  anorganic  substance;  that  is,  a  substance  made 
in  nature  only  by  life  processes.  It  is  the  same  kind  of 
substance  as  sugar  or  starch,  and  all  arc  known  as  carbohy- 
drates ;  that  is,  substances  composed  of  carbon,  and  of  hy- 
drogen and  oxygen  in  the  same  proportion  as  in  water. 
The  work  of  photosynthesis,  therefore,  is  to  form  carbohy- 
drates. The  carbohydrates,  such  as  sugar  and  starch,  rep- 
resent but  one  type  of  food  material.  Proteids  represent 
another  prominent  type,  substances  which  contain  carbon, 
hydrogen,  and  oxygen,  as  do  carbohydrates,  hut  which  also 
contain  other  elements,  notably  nitrogen,  sulphur,  and 
phosphorus.  The  white  of  an  egg  may  he  taken  as  an  ex- 
ample of  proteids.     They  seem  to  he  made  from  the  carbo- 


151  PLANT   RELATIONS. 

hydrates,  the  nitrogen,  sulphur,  and  other  necessary 
additional  elements  being  obtained  from  soil  substances 
dissolved  in  the  water  which  is  absorbed  and  conveyed 
to   the  leaves. 

113.  Transpiration. — The  water  which  is  absorbed  by  the 
roots  and  passes  to  the  leaves  is  much  more  abundant  than 
is  needed  in  the  process  of  photosynthesis.  It  should  be  re- 
membered that  the  water  is  not  only  used  as  a  raw  material 
for  food  manufacture,  but  also  acts  as  a  solvent  of  the  soil 
materials  and  carries  them  into  the  plant.  The  water  in 
excess  of  the  small  amount  used  in  food  manufacture  is 
given  off  from  the  plant  in  the  form  of  water  vapor,  the 
process  being  already  referred  to  as  transpiration  (see  §26). 

111.  Digestion. — Carbohydrates  and  proteids  may  be  re- 
garded as  prominent  types  of  plant  food  which  green 
plants  are  able  to  manufacture.  These  foods  are  trans- 
ported through  the  plant  to  regions  where  work  is  going  on, 
and  if  there  is  a  greater  supply  of  food  than  is  needed  for 
the  working  regions,  the  excess  is  stored  up  in  some  part 
of  the  plant.  As  a  rule,  green  plants  are  able  to  manufac- 
ture much  more  food  than  they  use,  and  it  is  upon  this  ex- 
cess that  other  plants  and  animals  live.  In  the  transfer  of 
foods  through  the  plant  certain  changes  are  often  neces- 
sary. For  example,  starch  is  insoluble,  and  hence  cannot 
be  carried  about  in  solution.  It  is  necessary  to  transform 
it  into  sugar,  which  is  soluble.  These  changes,  made  to 
facilitate  the  transfer  of  foods,  represent  digestion. 

115.  Assimilation. — When  food  in  some  form  has  reached 
a  working  region,  it  is  organized  into  the  living  substance 
of  the  plant,  known  as  protoplasm,  and  the  protoplasm 
builds  the  plant  structure.  This  process  of  organizing  the 
food  into  the  living  substance  is  known  as  assim  ilation. 

110.  Respiration. — The  formation  of  foods,  their  diges- 
tion and  assimilation  are  all  preparatory  to  the  process  of 
respiration,  which  may  be  called  the  use  of  assimilated 
food.     The  whole  working   power   of   the   plant    depends 


THE    NUTRITION    OF    PLANTS. 


155 


upon  respiration,  which  means  the  absorption  of  oxygen  by 
the  protoplasm,  fche  breaking  down  of  protoplasm,  and  the 
giving  off  of  carbon  dioxide  and  water  as  wastes.     The  im- 


Fig.  146.  The  common  Northern  pitcher  plant.  The  hollow  leaves,  each  with  a  hood 
and  a  wing,  form  a  rosette,  from  the  center  of  which  arise  the  flower  stalks.— 
After  Kerner. 


portance  of  this  process  may  be  realized  when  it  is  remem- 
bered that  there  is  the  same  need  in  our  own  living,  as  it 
is  essential  for  us  also  to  "breathe  in"  oxygen,  and  as  a 
result  we  " 'breathe  out  "  carbon  dioxide  and  water.  This 
breaking  down  or  "oxidizing"  of  protoplasm  releases  the 


156 


PLANT    RELATIONS. 


power  by  which  the  work  of  the  plant  is  carried  on  (see 
§27). 

117.  Summary  of  life-processes. — To  summarize  the  nu- 
tritive life-processes  in  green  plants,  therefore,  photosyn- 
thesis manufactures  carbohydrates, 
the  materials  used  being  carbon 
dioxide  and  water,  the  work  being 
done  by  the  chloroplast  with  the 
aid  of  light ;  the  manufacture  of 
proteids  uses  these  carbohydrates, 
and  also  substances  containing 
nitrogen,  sulphur,  etc.;  digestion 
puts  the  insoluble  carbohydrates 
and  the  proteids  into  a  soluble 
form  for  transfer  through  the 
plant ;  assimilation  converts  this 
food  material  into  the  living  sub- 
stance of  the  plant,  protoplasm  ; 
respiration  is  the  oxidizing  of  the 
protoplasm  which  enables  the 
plant  to  work,  oxygen  being  ab- 
sorbed, and  carbon  dioxide  and 
water  vapor  being  given  off  in 
the   process. 

118.  Plants  without  chlorophyll. 
— Eemembering  the  life-processes 
described  under  green  plants,  it  is 
evident  that  plants  without  chlo- 
rophyll cannot  do  the  work  of 
photosynthesis.  This  means  that 
they  cannot  manufacture  carbo- 
hydrates, and  that  they  must  de- 
pend upon  other  plants  or  animals  for  this  important  food. 
Mushrooms,  puff-balls,  moulds,  mildews,  rusts,  dodder, 
corpse  plants,  beech  drops,  etc.,  may  be  taken  as  illustra- 
tions of  such  plants. 


Fig.  147.  The  Southern  pitcher 
plant,  showing  the  funnelform 
and  winged  pitcher,  and  the 
overarching  hood  with  translu- 
cent spots.— After  Keener. 


THE   NUTRITION   OF   PLANTS. 


157 


Although  plants  without  chlorophyll  cannot  manufac- 
ture carbohydrates,  the  other  processes,  proteid  manufac- 
ture, digestion,  assimilation,  and  respiration,  are  carried  on. 
It  is  true,  however,  that  in  obtaining  carbohydrates  from 
other  plants  and  ani- 
mals, proteids  are  ob- 
tained also,  so  that 
proteid  manufacture 
is  not  so  prominent  as 
in  green  plants. 

119.  "Carnivorous" 
plants. — This  name  has 
been  given  to  plants 
which  have  developed 
the  curious  habit  of 
capturing  insects  and 
using  them  for  food. 
They  are  green  plants 
and,  therefore,  can  man- 
ufacture carbohydrates. 
But  they  live  in  soil 
poor  in  nitrogen  com- 
pounds, and  hence  pro- 
teid formation  is  inter- 
fered with.  The  bodies 
of  captured  insects  sup- 
plement the  proteid 
supply,  and  the  plants 
have  come  to  depend 
upon  them.  Many,  if 
not  all  of  these  carniv- 
orous plants,  secrete  a 


Fig.  148.  The  California]]  pitcher  plant  (Darling- 
toNia),  showing  twisted  and  winged  pitcher, 
the  overarching  hood  with  translucenl  spots, 
and    the    fish-tail    appendage   to    the    hood 

which   is   attractive   to  flying  insects.— After 
Kerner. 

which    acts    upon    the 

bodies  of   the   captured   insects  very  much  as  the   diges- 
tive substances  of  the  alimentary  canal  act  upon  proteids 


digestive   substance 


158 


PLANT   RELATIONS. 


swallowed  by  animals.      Some  common  illustrations  are  as 
follows  : 

(1)  Pitcher  plants. — In  these  plants  the  leaves  form 
tubes,  or  urns,  of  various  forms,  which  contain  water,  and 
to  which  insects  are  attracted  and  drowned  (see  Fig.  146). 
A  pitcher  plant  common  throughout  the  Southern  States 
may  be  taken  as  a  type  (see  Fig.  147).  The  leaves  are 
shaped  like  slender,  hollow  cones,  and  rise  in  a  tuft  from 

the  swampy  ground. 
The  mouth  of  this 
conical  urn  is  over- 
arched and  shaded 
by  a  hood,  in  which 
are  translucent  spots, 
like  small  windows. 
Around  the  mouth 
of  the  urn  a r  e 
glands,  which  se- 
crete a  sweet  liquid 
{nectar),  and  nectar 
drops  form  a  trail 
down  the  outside  of 
the  urn.  Inside,  just 
below  the  rim  of  the 
urn,  is  a  glazed  zone, 
so  smooth  that  insects 
cannot  walk  upon  it. 
Below  the  glazed  zone 
is  another  zone, 
thickly  set  with  stiff, 
hairs,  and  below  this  is  the  liquid  in 


Fig.    149. 


A  sun-dew,  showing  rosette  habit  of 
the  insect-catchine  leaves. 


downward-pointing 
the  bottom  of  the  urn. 

If  a  fly  is  attracted  by  the  nectar  drops  upon  this  curious 
leaf,  it  naturally  follows  the  trail  up  to  the  rim  of  the  urn, 
where  the  nectar  is  abundant.  If  it  attempts  to  descend 
within  the  urn,  it  slips  on  the  glazed  zone,  and  falls  into 


THE   NUTRITION    OF   PLANTS. 


159 


the  wtiter,  and  if  it  attempts  to  escape  by  crawling  up  the 
sides  of  the  urn,  the  thicket  of  downward-pointing  hairs 
prevents.  If  it  seeks  to  fly  away  from  the  rim,  it  flies 
towards  the  translucent  spots  in  the  hood,  which  look  like 
the  way  of  escape,  as  the  direction  of  entrance  is  in  the 
shadow  of  the  hood.  Pounding  against  the  hood,  the  fly 
falls  into  the  tube.     This  Southern  pitcher  plant  is  known 


nils  ww 

Fig.  150.  Two  leaves  of  a  sun-dew.  The  one  to  the  right  has  its  glandular  hairs 
fully  expanded  ;  the  one  to  the  left  shows  half  of  the  hairs  bending  inward,  in  the 
position  assumed  when  an  insect  has  been  captured.— After  Kerner. 


as  a  great  fly-catcher,  and  the  urns  are  often  well  supplied 
with  the  decaying  bodies  of  these  insects. 

A  much  larger  Californian  pitcher  plant  has  still  more 
elaborate  contrivances  for  attracting  insects  (see  Fig.  148). 

(2)  Drosera. — The  droseras  are  commonly  known  as 
"sun-dews,"  and  grow  in  swampy  regions,  the  leaves  form- 
ing small  rosettes  on  the  ground  (see  Fig.  140).  In  one 
form  the  leaf  blade  is  round,  and  the  margin  is  beset  by 
prominent  bristle-like  hairs,  each  with  a  globular  gland  at 
its  tip   (see   Fig.   150).     Shorter   gland-bearing   hairs   are 


160 


PLANT   RELATIONS. 


scattered  also  over  the  inner  surface  of  the  blade.  These 
glands  excrete  a  clear,  sticky  fluid,  which  hangs  to  them  in 
drops  like  dew-drops.     If  a  small  insect  becomes  entangled 


^W^KV-    -       '^-,.4^ 


mi&^^ 


Fig.  151.    Plants  of  Dionasa,  showing  the  rosette  habit  of  the  leaves  with  terminal 
traps,  and  the  erect  flowering  stem.— After  Kerner. 

in  the  sticky  drop,  the  hair  begins  to  curve  inward,  and 
presently  presses  its  victim  down  upon  the  surface  of  the 
blade.  In  the  case  of  larger  insects,  several  of  the  marginal 
hairs  may  join  together  in  holding  it,  or  the  whole  blade 
may  become  more  or  less  rolled  inward. 


THE   NUTRITION    OF   PLANTS. 


161 


(3)  Dioncea. — This  is  one  of  the  most  famous  and  re- 
markable of  fly-catching  plants  (see  Fig.  151).  It  is  found 
in  sandy  swamps  near  Wilmington,  North  Carolina.  The 
leaf  blade  is  constructed  like  a  steel  trap,  the  two  halves 
snapping  together,  and  the  marginal  bristles  interlocking 
like  the  teeth  of  a  trap  (see  Fig.  152).  A  few  sensitive 
hairs,  like  feelers,  are 
developed  on  the  leaf 
surface,  and  when  one 
of  these  is  touched  by 
a  small  flying  or  hover- 
ing insect,  the  trap 
snaps  shut  and  the  in- 
sect is  caught.  Only 
after  digestion  does  the 
trap  open  again. 

There  are  certain 
green  plants,  not  called 
carnivorous  plants, 
which  show  the  same 
general  habit  of  sup- 
plementing their  food 
supply,  and  so  reduc- 
ing the  necessity  of 
food  m  a  n  u  f  a  c  t  u  r  e . 
The  mistletoe  is  a 
green  plant,  growing 
upon  certain  trees,  from 
which  it  obtains  some  food 
is  able  to  manufacture. 

In  rich  soil,  the  organized  products  of  the  decaying 
bodies  of  plants  and  animals  are  often  absorbed  by  ordinary 
green  plants,  and  so  a  certain  amount  of  ready-made  food 
is  obtained. 


Fig.  152.  Three  leaves  of  Dioncea,  showing 
the  details  of  the  trap  in  the  leaves  to  right 
and  left,  and  the  central  trap  in  the  act  of 
capturing  an  insect. 


supplemei 


iting  that  whic 


h  it 


CHAPTER  XL 

PLANT  SOCIETIES:    ECOLOGICAL,  FACTORS. 

120.  Definition  of  plant  society.  — From  the  previous 
chapters  it  has  been  learned  that  every  complex  plant  is 
a  combination  of  organs,  and  that  each  organ  is  related  in 
some  special  way  to  its  environment.  It  follows,  therefore, 
that  the  whole  plant,  made  up  of  organs,  holds  a  very  com- 
plex relation  with  its  environment.  The  stem  demands 
certain  things,  the  root  other  things,  and  the  leaves  still 
others.  To  satisfy  all  of  these  demands,  so  far  as  possible, 
the  whole  plant  is  delicately  adjusted. 

The  earth's  surface  presents  very  diverse  conditions  in  ref- 
erence to  plant  life,  and  as  plants  are  grouped  according  to 
these  conditions,  this  leads  to  definite  associations  of  plants, 
those  adapted  to  the  same  general  conditions  being  apt  to 
live  together.  Such  an  association  of  plants  living  together 
in  similar  conditions  is  a  plant  society,  the  conditions  for- 
bidding other  plants.  It  must  not  be  understood  that  all 
plants  affecting  the  same  conditions  will  be  found  living 
together.  For  example,  a  meadow  of  a  certain  type  will  not 
contain  all  the  kinds  of  grasses  associated  with  that  type. 
Certain  grasses  will  be  found  in  one  meadow,  and  other 
grasses  will  be  found  in  other  meadows  of  the  same  type. 

Yery  closely  related  plants  generally  do  not  live  in  the 
same  society,  as  their  rivalry  is  apt  to  be  intense.  Closely 
related  plants  are  likely  to  occur,  however,  in  different 
societies  of  the  same  type.  A  plant  society,  therefore,  may 
contain  a  wide  representation  of  the  plant  kingdom,  from 
plants  of  low  rank  to  those  of  high  rank. 


PLANT   SOCIETIES:    ECOLOGICAL   FACTORS.  163 

Before  considering  some  of  the  common  societies,  it  is 
necessary  to  note  some  of  the  conditions  which  determine 
plant  societies.  Those  things  in  the  environment  of  the 
plant  which  influence  the  organization  of  a  society  are 
known  as  ecological  factors. 

121.  Water. — Water  is  certainly  one  of  the  most  im- 
portant conditions  in  the  environment  of  a  plant,  and  has 
great  influence  in  determining  the  organization  of  societies. 
If  all  plants  are  considered,  it  will  be  noted  that  the  amount 
of  water  to  which  they  are  exposed  is  exceedingly  variable. 
At  one  extreme  are  those  plants  which  are  completely 
submerged;  at  the  other  extreme  are  those  plants  of  arid 
regions  which  can  obtain  very  little  water  ;  and  between 
these  extremes  there  is  every  gradation  in  the  amount  of 
available  water.  Among  the  most  striking  adaptations  of 
plants  are  those  for  living  in  the  presence  of  a  great  amount 
of  water,  and  those  for  guarding  against  its  lack. 

One  of  the  first  things  to  consider  in  connection  with 
any  plant  society  is  the  amount  of  water  supply.  It  is  not 
merely  a  question  of  its  total  annual  amount,  but  of  its 
distribution  through  the  year.  Is  it  supplied  somewhat 
uniformly,  or  is  there  alternating  flood  and  drought  ?  The 
nature  of  the  water  supply  is  also  important.  Are  there 
surface  channels  or  subterranean  channels,  or  does  the 
whole  supply  come  in  the  form  of  rain  and  snow  which 
fall  upon  the  area  ? 

Another  important  fact  to  consider  in  connection  with 
the  water  supply  has  to  do  with  the  structure  of  the  soil. 
There  is  what  may  be  called  a  water  level  in  soils,  and  it  is 
important  to  note  the  depth  of  this  level  beneath  the  sur- 
face. In  some  soils  it  is  very  near  the  surface  ;  in  others, 
such  as  sandy  soils,  it  may  be  some  distance  beneath  the 
surface. 

Not  only  do  the  amount  of  water  and  the  depth  of  the 
water  level  help  to  determine  plant  societies,  but  also  the 
substances  which  the  water  contains.     Two  areas  may  have 


164  PLANT   RELATIONS. 

the  same  amount  of  water  and  the  same  water  level,  but 
if  the  substances  dissolved  in  the  water  differ  in  certain 
particulars,  two  entirely  distinct  societies  may  result. 

122.  Heat. — The  general  temperature  of  an  area  is  im- 
portant to  consider,  but  it  is  evident  that  differences  of 
temperature  are  not  so  local  as  differences  in  the  water 
supply,  and  therefore  this  factor  is  not  so  important  in  the 
organization  of  the  local  associations  of  plants,  called  socie- 
ties, as  is  the  water  factor.  In  the  distribution  of  plants 
over  the  surface  of  the  earth,  however,  the  heat  factor  is 
probably  more  important  than  the  water  factor.  The  range 
of  temperature  which  the  plant  kingdom,  as  a  whole,  can 
endure  during  active  work  may  be  stated  in  a  general  way 
as  from  0°  to  50°  C.  ;  that  is,  from  the  freezing  point  of 
water  to  122°  Fahr.  There  are  certain  plants  which  can 
work  at  higher  temperatures,  notably  certain  alga3  growing 
in  hot  springs,  but  they  may  be  regarded  as  exceptions.  It 
must  be  remembered  that  the  range  of  temperature  given 
is  for  plants  actively  at  work,  and  does  not  include  the  tem- 
perature which  many  plants  are  able  to  endure  in  a  specially 
protected  but  very  inactive  condition.  For  example,  many 
plants  of  the  temperate  regions  endure  a  winter  tempera- 
ture which  is  frequently  lower  than  the  freezing  point  of 
water,  but  it  is  a  question  of  endurance  and  not  of  work. 

It  must  not  be  supposed  that  all  plants  can  work  equally 
well  throughout  the  Avhole  range  of  temperature  given,  for 
they  differ  Avidely  in  this  regard.  Tropical  plants,  for  in- 
stance, accustomed  to  a  certain  limited  range  of  high  tem- 
perature, cannot  work  continuously  at  the  lower  tempera- 
tures. For  each  kind  of  plant  there  is  what  may  be  called 
a  zero  point,  below  which  it  is  not  in  the  habit  of  working. 

AVhile  it  is  important  to  note  the  general  temperature 
of  an  area  throughout  the  year,  it  is  also  necessary  to  note 
its  distribution.  Two  regions  may  have  presumably  the 
same  amount  of  heat  through  the  year,  but  if  in  the  one  case 
it  is  uniformly  distributed,  and  in  the  other  great  extremes 


PLANT   SOCIETIES:    ECOLOGICAL    FACTORS.  165 

of  temperature  occur,  the  same  plants  will  not  be  found  in 
both.  It  is,  perhaps,  most  important  to  note  the  tempera- 
ture during  certain  critical  periods  in  the  life  of  plants, 
such  as  the  flowering  period  of  seed-plants. 

Although  the  temperature  problem  may  be  compara- 
tively uniform  over  any  given  area,  the  effect  of  it  may  be 
noted  in  the  succession  of  plants  through  the  growing  sea- 
son. In  our  temperate  regions  the  spring  plants  and  summer 
plants  and  autumn  plants  differ  decidedly  from  one  another. 
It  is  evident  that  the  spring  plants  can  endure  greater 
cold  than  the  summer  plants,  and  the  succession  of  flowers 
will  indicate  someAvhat  these  relations  of  temperature. 

It  should  be  remarked,  also,  that  not  only  is  the  tem- 
perature of  the  air  to  be  noted,  but  also  that  of  the  soil. 
These  two  temperatures  may  differ  by  several  degrees,  and 
the  soil  temperature  especially  affects  root  activity,  and 
hence  is  a  very  important  factor  to  discover. 

At  this  point  it  is  possible  to  call  attention  to  the  effect 
of  the  combination  of  ecological  factors.  For  instance,  in 
reference  to  the  occurrence  of  plants  in  any  society,  the 
water  factor  and  the  heat  factor  cannot  be  considered  each 
by  itself,  but  must  be  taken  in  combination.  For  example, 
if  in  a  given  area  there  is  a  combination  of  maximum  heat 
and  minimum  water,  the  result  will  be  a  desert,  and  only 
certain  specially  adapted  plants  can  exist.  It  is  evident 
that  the  great  heat  increases  the  transpiration,  and  tran- 
spiration when  the  supply  of  water  is  very  meager  is  pe- 
culiarly dangerous.  Plants  which  exist  in  such  conditions, 
therefore,  must  be  specially  adapted  for  controlling  tran- 
spiration. On  the  other  hand,  if  in  any  area  the  combina- 
tion is  maximum  heal  and  maximum  water,  the  result  will 
be  the  most  luxuriant  vegetation  on  the  earth,  such  as  grows 
in  the  rainy  tropics.  It  is  evident  that  the  possible  com- 
binations of  the  water  and  heat  factors  may  be  very  numer- 
ous, and  that  it  is  the  combination  which  chiefly  affects 
plant  societies. 


166  PLANT   RELATIONS. 

123.  Soil. — The  soil  factor  is  not  merely  important  to 
consider  in  connection  with  those  plants  directly  related 
to  the  soil,  but  is  a  factor  for  all  plants,  as  it  determines 
the  substances  which  the  water  contains.  There  are  two 
things  to  be  considered  in  connection  with  the  soil,  namely, 
its  chemical  composition  and  its  physical  properties.  Per- 
haps the  physical  properties  are  more  important  from  the 
standpoint  of  soil-related  plants  than  the  chemical  com- 
position, although  both  the  chemical  and  physical  nature 
of  the  soil  are  so  bound  up  together  that  they  need  not  be 
considered  se]^arately  here.  The  physical  properties  of  the 
soil,  which  are  important  to  plants,  are  chiefly  those  which 
relate  to  the  water  supply.  It  is  always  important  to  de- 
termine how  receptive  a  soil  is.  Does  it  take  in  water 
easily  or  not  ?  It  is  also  necessary  to  determine  how  re- 
tentive it  is  ;  it  may  receive  water  readily,  but  it  may  not 
retain  it. 

For  convenience  in  ordinary  field  work  with  plants, 
soils  may  be  divided  roughly  into  six  classes  :  (1)  rock, 
which  means  solid  uncrumbled  rock,  upon  which  certain 
plants  are  able  to  grow  ;  (2)  sand,  which  has  small  water 
capacity,  that  is,  it  may  receive  water  readily  enough,  but 
does  not  retain  it ;  (3)  lime  soil ;  (4)  clay,  which  has  great 
water  capacity  ;  (5)  humus,  which  is  rich  in  the  products 
of  plant  and  animal  decay  ;  (0)  salt  soil,  in  which  the  water 
contains  various  salts,  and  is  generally  spoken  of  as  alka- 
line. These  divisions  in  a  rough  way  indicate  both  the 
structure  of  the  soil  and  its  chemical  composition.  Not 
only  should  the  kinds  of  soil  on  an  area  be  determined, 
liii t  their  depth  is  an  important  consideration.  It  is 
very  common  to  find  one  of  these  soils  overlying  another 
one,  and  this  relation  between  the  two  will  have  a  very 
important  effect.  For  instance,  if  a  sand  soil  is  found 
lying  over  a  clay  soil,  the  result  will  be  that  the  sand  soil 
will  retain  far  more  water  than  it  would  alone.  If  a  humus 
soil  in  one  area  overlies  a  sand  soil,  and  in  another  area 


PLANT    SOCIETIES:    ECOLOGICAL    FACTORS.  167 

overlies  a  clay  soil,  the  humus  will  differ  very  much  iu  the 
two  cases  in  reference  to  water. 

The  soil  cover  should  also  be  considered.  The  common 
soil  covers  are  snow,  fallen  leaves,  and  living  plants.  It 
will  be  noticed  that  all  these  covers  tend  to  diminish  the 
loss  of  heat  from  the  soil,  as  well  as  the  access  of  heat  to 
the  soil.  In  other  words,  a  good  soil  cover  will  very  much 
diminish  the  extremes  of  temperature.  All  this  tends  to 
increase  the  retention  of  water. 

l'U.  Light. — It  is  known  that  light  is  essential  for  the 
peculiar  work  of  green  plants.  However,  all  green  plants 
cannot  have  an  equal  amount  of  light,  and  some  have 
learned  to  live  with  a  less  amount  than  others.  While 
no  sharp  line  can  be  drawn  between  green  plants  which 
use  intense  light,  and  those  which  use  less  intense  light, 
we  still  recognize  in  a  general  way  what  are  called  light 
plants  and  shade  plants.  We  know  that  certain  plants 
are  chiefly  found  in  situations  where  they  can  be  exposed 
freely  to  light,  and  that  other  plants,  as  a  rule,  are  found 
in  shady  situations. 

Starting  with  this  idea,  we  find  that  plants  grow  in 
strata.  In  a  forest  society,  for  example,  the  tall  trees  rep- 
resent the  highest  stratum  ;  below  this  there  may  be  a 
stratum  of  shrubs,  then  tall  herbs,  then  low  herbs,  then 
forms  like  mosses  and  lichens  growing  close  to  the  ground. 
In  any  plant  society  it  is  important  to  note  the  number  of 
these  strata.  It  may  be  that  the  highest  stratum  shades 
so  densely  that  many  of  the  other  strata  are  not  represented 
at  all.  An  illustration  of  this  can  be  obtained  from  a 
dense  beech  forest. 

125.  Wind.— It  is  generally  known  that  Mind  has  a  dry- 
ing effect,  and,  therefore,  it  increases  the  transpiration  of 
plants  and  tends  to  impoverish  them  in  water.  This  factor 
is  especially  conspicuous  in  regions  where  there  are  pre- 
vailing winds,  such  as  near  the  sea-coast,  around  the  great 
lakes,  and  on  the  prairies  and  plains.     In  all  such  regions 


168  PLANT   RELATIONS. 

the  plants  have  been  compelled  to  adapt  themselves  to  this 
loss  of  water  ;  and  in  some  regions  the  prevailing  winds  are 
so  constant  and  violent  that  the  force  of  the  wind  itself  has 
influenced  the  appearance  of  the  vegetation,  giving  what  is 
called  a  characteristic  physiognomy  to  the  area. 

These  five  factors  have  been  selected  from  a  much  larger 
number  that  might  be  enumerated,  but  they  may  be  re- 
garded as  among  the  most  important  ones.  It  will  be 
noticed  that  these  factors  may  be  combined  in  all  sorts 
of  ways,  so  that  an  almost  endless  series  of  combinations 
seems  to  be  possible.  This  will  give  some  idea  as  to  the 
possible  number  of  plant  societies,  for  they  may  be  as 
numerous  as  are  the  combinations  of  these  factors. 

126.  The  great  groups  of  societies. — It  is  possible  to  re- 
duce the  very  numerous  societies  to  three  or  four  great 
groups.  For  convenience,  the  water  factor  is  chiefly  used 
for  this  classification.  It  results  in  a  convenient  classifica- 
tion, but  one  that  is  probably  more  or  less  artificial.  The 
selection  of  any  one  factor  from  among  the  many  for  the 
purpose  of  classification  never  results  in  a  very  natural 
classification  when  the  combination  of  factors  determines 
the  group.  However,  for  general  purposes,  the  usual 
classification  on  the  basis  of  water  supply  will  be  used. 
On  this  basis  there  are  three  great  groups  of  societies, 
as  follows  : 

(1)  Hydrophytes. — The  name  means  "water  plants/'  and 
suggests  that  such  societies  are  at  that  extreme  of  the  water 
supply  where  it  is  \ cry  abundant.  Such  plants  may  grow 
in  the  water,  or  in  very  wet  soil,  but  in  any  event  they  are 
exposed  to  a  large  amount  of  water. 

(2)  Xerophytes. — The  name  means  "drouth  plants," 
and  suggests  the  other  extreme  of  the  water  supply.  True 
xerophytes  are  exposed  to  dry  soil  and  dry  atmosphere. 

(3)  Mesophytes. — Between  the  two  extremes  of  the  water 
supply  there  is  a  great  middle  region  of  medium  water 
supply,  and  plants  which  occupy  it  are  known  as  meso- 


PLANT   SOCIETIES:    ECOLOGICAL   FACTORS.  169 

phytes,  the  plants  of  medium  conditions.  It  is  evident  that 
mesophytes  gradually  pass  into  hydrophytes  on  the  one 
side,  and  into  xerophytes  on  the  other ;  but  it  is  also  evi- 
dent that  mesophyte  societies  have  the  greatest  range  of 
water  supply,  extending  from  a  large  amount  of  water  to 
a  very  small  amount. 

It  should  be  understood  that  these  three  groups  of  socie- 
ties, which  are  distinguished  from  one  another  by  the  amount 
of  the  water  supply,  are  artificial  groups  rather  than  natural 
ones,  for  they  bring  together  unrelated  societies,  and  often 
separate  those  that  are  closely  related.  For  example,  a 
swampy  meadow  is  put  among  hydrophyte  societies  by  this 
classification  ;  and  it  may  shade  into  an  ordinary  meadow, 
which  belongs  among  the  mesophytes.  Probably  the  largest 
fact  which  may  be  used  in  grouping  plant  societies  is  that 
certain  societies  are  so  situated  that  they  seek  for  the  most 
part  to  reduce  transpiration,  and  that  others  are  so  situated 
that  they  seek  for  the  most  part  to  increase  transpiration. 

However,  the  factors  which  determine  societies  are  so 
numerous  that  they  cannot  be  presented  in  an  elementary 
book,  and  the  simpler  artificial  grouping  given  above  will 
serve  to  introduce  the  societies  to  observation. 


CHAPTER   XII. 

HYDROPHYTE    SOCIETIES. 

127.  General  character. —  Hydrophytes  are  related  to 
abundant  water,  either  throughout  their  whole  structure 
or  in  part  of  their  structure.  It  is  a  well-known  fact  that 
hydrophytes  are  among  the  most  cosmopolitan  of  plants, 
and  hydrophyte  societies  in  one  part  of  the  world  look 
very  much  like  hydrophyte  societies  in  any  other  region. 
It  is  probable  that  the  abundant  water  makes  the  condi- 
tions more  uniform. 

It  is  evident  that  for  those  plants,  or  plant  parts,  which 
are  submerged,  the  water  affects  the  heat  factor  by  dimin- 
ishing the  extremes.  It  also  affects  the  light  factor,  in  so 
far  as  the  light  must  pass  through  the  water  to  reach  the 
chlorophyll-containing  parts,  as  light  is  diminished  in 
intensity  by  passing  through  the  water.  Before  consider- 
ing a  few  hydrophyte  societies,  it  is  necessary  to  note  the 
prominent  hydrophyte  adaptations. 

128.  Adaptations, — In  order  that  the  illustration  may  be 
as  simple  as  possible,  a  complex  plant  completely  exposed 
to  water  is  selected,  for  it  is  evident  that  the  relations  of  a 
swamp  plant,  with  its  roots  in  water  and  its  stem  and  leaves 
exposed  to  air,  are  complicated.  A  number  of  adaptations 
may  be  noted  in  connection  with  the  submerged  or  floating 
plant. 

(1)  Tliin-icalled  epidermis. — In  the  case  of  the  soil-re- 
lated plants,  the  water  supply  comes  mainly  from  the  soil, 
and  the  root  system  is  constructed  to  absorb  it.  In  the 
case  of  the  water  plant  under  consideration,  however,  the 


HYDROPHYTE    SOCIE  TIES. 


171 


whole  plant  body  is  exposed  to  the  water  supply,  aud  there- 
fore absorption  may  take  place  through  the  whole  surface 
rather  than  at  any  particular  region  such  as  the  root.  In 
order  that  this  may  be  done,  however,  it  is  necessary  for 
the  epidermis  to  have  thin  walls,  which  is  usually  not  the 
case  in  epidermis  exposed 
to  the  air,  where  a  certain 
amount  of  protection  is 
needed  in  the  way  of 
thickening. 

(2)  Roots  much  reduced 
or  tvanting.— It  must  be 
evident  that  if  water  is 
being  absorbed  by  the 
whole  free  surface  of  the 
plant,  there  is  not  so 
much  need  for  a  special 
root  region  for  absorp- 
tion. Therefore,  in  such 
water  plants  the  root  sys- 
tem may  be  much  re- 
duced, or  may  even  disap- 
pear entirely.  It  is  often 
retained,  however,  to  act 
as  a  holdfast,  rather  than 
as  an  absorbent  organ,  for 
most  water  plants  anchor 
themselves  to  some  sup- 
port. 

(3)  Reduction  of  water-conducting  tissues. — In  the  ordi- 
nary soil-related  plants,  not  only  is  an  absorbing  root  sys- 
tem necessary,  but  also  a  conducting  system,  to  carry  the 
water  absorbed  from  the  roots  to  the  leaves  and  elsewhere. 
It  has  already  been  noted  that  this  conducting  system  takes 
the  form  of  woody  strands.  It  is  evident  that  if  water 
is  being  absorbed  by  the  whole  surface  of  the  plant,  the 


Fig.  153.  Fragment  of  a  common  seaweed 
{Fucks),  showing  the  body  with  forking 
branching  and  bladder-like  air  cavities.— 
After  LtKKssKN. 


172 


PLANT   RELATION'S. 


work  of  conduction  is  not  so  extensive  or  definite,  and 
therefore  in  such  water  plants  the  woody  bundles  are  not 
so  prominently  developed  as  in  land  plants. 

(4)   Reduction    of  mechanical  tissues. — In   the   case   of 
ordinary  land  plants,  certain  firm  tissues  are  developed  so 


Fig.  154.    Gulf  weed  (Sargassmn),  showing  the  thallus  differentiated  into  stem-like  and 
leaf-like  portions,  and  also  the  bladder-like  floats. — After  Bennett  and  Murray. 


that  the  plant  may  maintain  its  form.  These  supporting 
tissues  reach  their  culmination  in  such  forms  as  trees, 
where  massive  bodies  are  able  to  stand  upright.  It  is  evi- 
dent that  in  the  water  there  is  no  such  need  for  rigid  sup- 
porting tissues,  as  the  buoyant  power  of  water  helps  to 
support  the  plant.     This  fact  may  be  illustrated  by  taking 


HYDROPHYTE    SOCIETIES. 


173 


out  of  water  submerged  plants  which  seem  to  be  upright, 
with  all  their  parts  properly  spread  out.   When  removed  they 
collapse,  not  being  able  to  support  themselves  in  any  way. 
(5)  Development  of  air  cavities. — The  presence  of  air  in 
the  bodies  of  water  plants  is  necessary  for  two  reasons:  (1), 


-■< 


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i*k 


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,*3$: 


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jK 


Fig.  155.  Bladderwort,  showing  the  numerous  bladders  which  float  the  plant,  the 
finely  divided  water  leaves,  and  the  erect  flowering  stems.  The  bladders  are  also 
effective  "insect  traps,"  Utricularia  being  one  of  the  "carnivorous  plants.'" 
—After  Kerner. 


to  aerate  the  plant  ;  (2),  to  increase  its  buoyancy.  In  most 
complex  water  plants  there  must  be  some  arrangement  for 
the  distribution  of  air  containing  oxygen.  This  usually 
takes  the  form  of  air  chambers  and  passageways  in  the 
body  of  the  plant  (see  Figs.  87,  88,  89,  15G).  Of  course 
such  air  chambers  increase  the  buoyancy  of  the  body. 
Sometimes,  however,  a  special  buoyancy  is  provided  for 
by  the  development  of  regular  floats,  which  are  bladder- 


lTJr  PLANT   RELATIONS. 

like  bodies  (see  Figs.  L53,  154).  These  floats  are  very  com- 
mon among  certain  of  the  seaweeds,  and  are  found  among 
higher  plants,  as  the  utricularias  or  bladderworts,  which 
have  received  their  name  from  the  numerous  bladders  de- 
veloped in  connection  with  their  bodies  (see  Fig.  155). 

129.  The  two  groups  of  societies. — The  hydrophyte  so- 
cieties may  be  put  into  two  great  divisions.  True  hydro- 
phytes  are  those  in  which  the  contents  and  temperature  of 
the  water  are  favorable  to  plant  activity ;  while  xeroplnjtic 
hydrophytes  are  those  in  which  the  contents  and  tempera- 
ture of  the  water  are  unfavorable  to  plant  activity,  and  the 
structures  of  the  plants  are  adapted  to  reduce  transpiration, 
resembling  in  this  feature  the  structures  displayed  by  the 
true  xerophytes  (see  §155). 

I.  Tkue  hydrophytes. 
A.  Free-sivimming  societies. 

130.  Definition. — In  these  societies  there  is  the  largest 
exposure  to  water,  and  no  relation  at  all  to  the  nutrient  or 
mechanical  support  of  the  soil,  the  plants  being  completely 
supported  by  the  water.  They  may  be  either  submerged 
or  floating,  and  they  are  free  to  move  either  by  locomo- 
tion or  by  water  currents.  Two  prominent  societies  are 
selected  as  types. 

131.  The  plankton.— This  term  is  used  to  designate  the 
minute  organisms,  both  plants  and  animals,  which  are 
found  in  the  water.  The  plankton  is  composed  of  indi- 
viduals invisible  to  the  naked  eye,  but  taken  together  they 
represent  an  enormous  organic  mass.  The  plankton  socie- 
ties are  especially  well  represented  in  the  colder  oceanic 
waters,  but  they  are  not  absent  from  any  waters.  Among 
the  most  prominent  plants  in  these  societies  are  the  dia- 
toms. Diatoms  are  minute  plants  of  various  forms,  and  all 
have  a  wall  very  full  of  silica.      This  makes  their  bodies 


HYDROPHYTE    SOCIETIES. 


175 


extremely  enduring,  and  therefore  diatoms  are  often  found 
in  great  deposits  in  the  rocks,  in  some  cases  forming  the 
whole  mass  of  rock.  Associated  with  the  diatoms  are 
numerous  other  plant  and  animal  forms. 

132.  Pond  societies. — The  word  pond  is  used  to  indicate 
stagnant  or  slow-moving  waters.  In  such  waters  free- 
swimming  plants  of  all  groups  are  associated.  Of  course 
the  algaa  are  well  represented,  but  even  the  highest  plants 
a  re  repre- 
sented by  the  0 
duckweeds,  0 
which  are  very 
com  m  o  n  1  y 
seen  in  the 
form  of  small 
green  disks 
floating  on  the 
surface  of  the 
water,  which 
they  frequent- 
ly cover  with 
great   masses 

(see  Fig.  156).  It  should  be  observed  that  the  floating  and 
submerged  positions  result  in  a  difference  in  light-relations. 
The  floating  forms  may  be  regarded  as  light  forms,  being 
exposed  to  the  greatest  amount  of  light.  The  submerged 
forms  are  shade  plants,  and  the  shading  becomes  greater 
as  the  depth  of  the  water  is  greater.  It  must  not  be  sup- 
posed that  submerged  plants  can  live  at  any  depth,  for 
soon  a  limit  is  reached,  beyond  which  the  light  is  not 
intense  enough  to  enable  plants  to  work. 

It  has  been  noticed  that  this  complete  water  habit  has 
affected  plants  in  many  ways.  For  instance,  the  duck- 
weeds are  related  to  land  plants  with  root,  stem,  and  leaves, 
but  they  have  lost  the  distinction  between  stem  and  leaf, 
and  the  body  is  merely  a  flat  leaf-like  disk  floating  upon 


Fig.  156.  A  section  through  the  body  of  a  duckweed  (Lem  a  a). 
showing  the  air  spaces  (a)  which  make  it  buoyant,  the 
origin  (/•)  of  the  simple  dangling  root,  and  the  pockets 
(s  and  I)  from  which  new  plants  bud  out,  and  in  which 
flowers  are  developed. 


176  PLANT    RELATIONS. 

the  water,  with  a  few  roots  dangling  from  the  under  side, 
or  with  no  roots  at  all  (see  Fig.  J 50).  This  same  duck- 
weed also  shows  some  interesting  modifications  in  its  hab- 
its of  reproduction.  Although  related  to  plants  which  pro- 
duce flowers  and  make  seed,  the  duckweeds  have  almost 
lost  the  power  of  producing  flowers,  and  when  they  do 
produce  them,  seeds  are  very  seldom  formed.  In  other 
words,  the  ordinary  method  of  reproduction  employed 
by  flowering  plants  has  heen  more  or  less  abandoned. 
Replacing  this  method  of  reproduction  is  a  great  power 
of  vegetative  propagation.  From  the  disk-like  body  of 
the  plant  other  disk-like  bodies  bud  out,  and  this  bud- 
ding continues  until  a  large  group  of  disks,  more  or 
less  connected  with  each  other,  may  be  formed.  These 
plants  also  form  what  are  known  as  winter  buds — well 
protected  bud-like  bodies  which  sink  to  the  bottom  of 
the  pond  when  the  floating  plants  are  destroyed,  and 
remain  protected  by  the  mucky  bottom  until  the  waters 
become  warm  again  in  the  next  growing  season. 

In  examining  the  pond  societies,  therefore,  attention 
should  be  paid  to  the  floating  forms  and  the  submerged 
forms,  and  also  to  the  varying  depths  of  the  latter.  It  will 
also  be  noted  that  the  leaves  of  floating  forms  are  com- 
paratively broad,  while  those  of  submerged  forms  are 
narrow. 

B.   Anchored  societies. 

133.  Definition. — These  are  societies  fixed  to  the  soil  but 
with  submerged  or  floating  leaves.  In  this  case  there  is 
still  great  exposure  to  water,  but  there  is  also  a  definite  soil 
relation.  Two  prominent  societies  are  selected  from  this 
group  for  illustration. 

134.  Rock  societies. — The  term  rock  is  used  in  this  con- 
nection in  a  very  general  way,  meaning  simply  some  firm 
support  beneath  the  water  ;  it  is  just  as  likely  to  be  a  stick 


HYDROPHYTE    SOCIETIES. 


i  I 


as  a  stone.  Probably  the  most  prominent  group  of  plants 
affecting  these  conditions  are  alga?,  both  fresh  water  and 
marine.     In  the  fresh  waters  very  many  of  the  alga?  will  be 


** 

:* 


sf^ 


Fig.  157.    A  group  of  marine  seaweeds  (Laminarias).    Note  the  various  habits  of  the 
plant  body  and  the  root-like  holdfasts.— After  Kerner. 

found  anchored  to  some  support,  The  largest  display  of 
such  forms,  however,  is  found  among  the  marine  alga?, 
which  abound  along  all  seacoasts  (see  Fig.    157).     It  will 


Fig.  158.  A  natural,  but  nearly  overgrown  lily  pond.  The  lily  pads  may  be  seen 
rising  more  or  less  above  the  water  where  they  are  thickest.  The  forest  growth  in 
the  background  is  probably  a  tamarack  (larch)  swamp.  It  is  to  be  noticed  that  as 
the  lily  pond  loses  its  water  it  is  being  invaded  by  the  coarse  sedge  and  grass 
growth  of  a  swamp-moor.  Between  the  lily  pond  and  the  forest  is  a  swamp- 
thicket.  At  least  four  distinct  societies  are  represented  in  this  view.  A  fifth  is 
probably  represented  in  the  form  of  plants  of  the  reed-swamp  type,  which  form  a 
transition  between  the  lily  pond  and  the  swamp-thicket. 


HYDROPHYTE   SOCIETIES.  179 

be  noticed  that  the  habit  of  anchorage  demands  the 
development  of  special  organs  of  attachment,  which  usu- 
ally take  the  form  of  root-like  structures,  often  associated 
with  sucker-like  disks.  Associated  with  the  anchoring 
structures  is  often  a  development  of  floats,  which  is  es- 
pecially characteristic  of  seaweeds,  enabling  the  working 
body  to  float  freely  in  the  water  (see  Figs.  153,  154).  It  is 
evident  that  while  free-swimming  forms  may  be  suitable 
for  stagnant  waters,  anchored  forms  are  better  adapted  for 
moving  waters.  Therefore,  where  there  are  currents  of 
water,  or  wave  action,  the  anchored  forms  predominate. 
The  ability  to  live  in  moving  waters,  and  often  in  those 
that  become  violently  agitated,  has  its  advantage  to  the 
plant  in  the  more  rapidly  renewed  food  material.  In  such 
a  situation  free-swimming  forms  would  soon  be  stranded 
or  disposed  of  in  quieter  waters. 

In  the  case  of  the  marine  seaweeds  there  is  an  interest- 
ing relation  between  the  depth  of  the  water  and  the  color 
of  the  plants.  While  the  fresh  water  algae  are  prevailingly 
green,  it  will  be  remembered  that  the  prevailing  colors  of 
the  alga3  of  the  seashore  are  brown  and  red.  The  brown 
often  passes  into  some  shade  of  yellow,  and  the  red  may 
merge  into  purple  or  violet,  but  in  general  the  two  types  of 
color  may  be  called  brown  and  red.  It  has  been  noticed 
that  the  brown  forms  are  found  at  less  depth  than  the  red 
forms,  so  that  in  a  general  way  there  are  two  zones  of  dis- 
tribution in  relation  to  depth,  the  red  zone  being  the  lower 
one  and  the  yellow  zone  the  upper.  Just  what  this  means 
in  the  economy  of  the  plants  is  not  clear,  but  it  has  been 
suggested  that  the  yellow  and  the  red  colors  assist  the 
chlorophyll  in  its  work,  which  is  more  or  less  interfered 
with  by  the  diminished  intensity  of  the  lighl  passing 
through  sea  water. 

135.  Loose  soil  societies. — This  phrase  is  used  merely  to 
contrast  with  rock  societies,  referring  to  the  fact  that  the 
anchorage  is  not  merely  for  mechanical  support,  but  that 


HYDROPHYTE    SOCIETIES.  181 

there  is  a  definite  relation  to  soil  in  which  roots  or  root-like 
structures  are  embedded.  Societies  of  this  type  contain 
the  greatest  variety  of  plants  of  all  ranks.  In  these  soci- 
eties are  found  alga?,  mosses,  fern  plants,  pondweeds, 
water  lilies,  etc.  (see  Figs.  158,  159,  160,  161).  Pondweeds 
and  water  lilies  may  be  taken  as  convenient  types  of  high 
grade  plants  which  grow  in  such  conditions. 

In  the  first  place,  it  will  be  noticed  that  they  are  in- 
clined to  social  growths,  great  numbers  of  individuals 
groAving  together  and  forming  what  are  known  as  lily 
ponds  or  pondweed  beds,  although  in  the  small  lakes  of 
the  interior  where  pondweeds  abound  in  masses,  they  are 
more  commonly  known  as  "  pickerel  beds."  If  the  petiole 
of  a  lily  pad  be  traced  down  under  the  water,  it  will  be 
found  to  arise  from  an  intricate  mass  of  thick,  knotted 
stems.  So  extensively  do  these  stems  (rootstocks)  in  the 
mucky  bottom  branch  that  they  are  able  to  give  rise  to 
close  set  masses  of  leaves. 

Water  lilies  and  pondweeds  may  also  be  compared  to 
show  the  effect  of  the  floating  habit  in  contrast  with  the 
submerged  habit.  The  leaves  of  water  lilies  float  on  the 
surface,  and  therefore  are  broad  ;  and  being  exposed  to  light 
are  a  vivid  green,  indicating  the  abundant  development  of 
chlorophyll.  Many  of  the  pondweeds,  however,  are  com- 
pletely submerged.  As  one  floats  over  one  of  these  "  pick- 
erel beds,"  the  leafy  plants  maybe  seen  at  considerable 
depths,  and  have  a  pallid,  translucent  look.  It  will  be 
seen  that  in  these  cases  the  leaf  forms  are  narrow  rather 
than  broad,  often  being  ribbon-like,  or  in  some  submerged 
plants  even  cut  up  into  thread-like  forms.  It  is  evident 
that  such  narrow  leaf  forms  can  respond  more  easily  to 
water  movements  than  broad  forms.  The  pallid  look  of 
these  submerged  leaves  indicates  that  there  lias  not  been 
an  abundant  development  of  chlorophyll.  Sonic  pondweeds, 
however,  have  both  types  of  leaves,  some  being  submerged 
and  others  floating.    In  these  cases  it  is  interesting  to  notice 


Fig.  160.— A  group  of  pondweeds.  The  stems  are  sustained  in  an  erect  position  by 
the  water,  and  the  narrow  leaves  are  exposed  to  a  light  whose  intensity  is  dimin- 
ished by  passing  through  the  water.— After  Keener. 


HYDROPHYTE   SOCIETIES.  183 

the  corresponding  change  of  form  ;  on  the  same  individual 
the  submerged  leaves  are  very  narrow,  or  divided  into 
very  narrow  lobes,  while  the  floating  ones  are  broad 
(see  Fig.  162).  The  relation  of  the  plant  to  the  water, 
therefore,  has  determined  the  leaf  form.  The  advantage 
of  the  floating  habit  of  leaves  is  not  merely  a  better  rela- 
tion to  light,  but  the  carbon  dioxide  used  in  photosynthesis 
and  the  oxygen  used  in  respiration  may  be  obtained  freely 
from  the  air,  rather  than  from  the  water.  It  will  also  be 
noticed  that  these  water  plants  usually  send  their  flowers 
to  the  surface,  indicating  that  such  a  position  is  more  fav- 
orable for  the  work  of  the  flower  than  a  submerged  position. 
Any  society  of  this  type  will  furnish  abundant  material  for 
observation,  and  it  is,  perhaps,  the  most  valuable  type  of 
society  for  study  that  has  been  mentioned  so  far. 

C.   Sivamp  societies. 

136.  Definition.— In  swamp  societies  the  plants  are  rooted 
in  water,  or  in  soils  rich  in  water,  but  the  stems  bearing 
the  leaves  rise  above  the  surface.    Among  the  hydrophytes, 
swamp  plants  are  least  exposed  to  water,  and  as  the  stem 
and  its  leaves  are  exposed  to  the  air,  there  is  no  such  reduc- 
tion of  the  root  system  and  of  conducting  and  mechanical 
tissues  as  in  the  other  hydrophytes.     Also  the  epidermis  is 
not  thin,  and  there  is  no  development  of  floats  to  increase 
the    buoyancy.     However,  the  root  must  be  aerated,  and 
hence  air  chambers  and  passageways  are  abundant.     This 
aeration  of  the  root  system  reaches  a  very  high  develop- 
ment in  such  swamp  trees  as  the  cypress.    In  cypress  swamps 
the  so-called  "knees"  are  abundant,  and  they  are  found  to 
be  special  growths  from  the  root  system,  which  rise  above 
the  surface  of  the  water,  both  for  bracing  and  to  admil   air 
to  the  roots  (see  Fig.  91).     It  has  been  shown  that  if  such 
swamps  are  flooded  above  the  level  of  the  knees,  many  of 
the  trees  are  killed.     In  ordinary  cases  the  air  is  admitted 
13 


*? 


tg&»    X  <&. . 


Mil 


Fig.  101.  Eel  grass  (Vallisneria),  a  common  pondweed  plant.  The  plants  are 
anchored  and  the  foliage  is  submerged.  The  carpel-bearing  flowers  are  carried  to 
the  surface  on  long  stalks  which  allow  a  variable  depth  of  water.  The  stamen- 
bearing  flowers  remain  submerged,  as  indicated  near  the  lower  left  corner,  the 
flowers  breaking  away  and  rising  to  the  surface,  where  they  float  and  effect  pollina- 
tion.—After  Keuner. 


HYDKOPHYTE   SOCIETIES.  185 

through  openings  in  the  epidermis  of  the  stem  and  leaves, 
and  so  enters  the  air  passageways  and  reaches  the  roots. 
Another  habit  of  swamp  plants  is  called  turf-building, 
which  means  that  new  individuals  arise  from  older  ones, 
and  so  a  dense  mat  of  roots  and  rootstocks  is  formed.  Very 
prominent   among   these   turf-building   swamp  plants  are 


Fig.  162.  Two  leaves  of  a  water  buttercup,  showing  the  difference  in  the  forms  of 
submerged  and  aerial  leaves  on  the  same  plant,  the  former  being  much  more 
finely  divided.— After  Strasburger. 

the  sedges.     Some  of  the  prominent  swamp  societies  may  be 
enumerated  as  follows  : 

137.  Reed  swamps.— The  reed-swamp  plants  are  tall  wand- 
like forms,  which  grow  in  rather  deep,  still  water  (see  Fig. 
163).  Prominent  as  types  are  the  cat-tail  flag,  bulrushes, 
and  reed  grasses.  Such  an  assemblage  of  forms  usually 
characterizes  the  shallow  margins  of  small  lakes  and  ponds. 
In  such  places  the  different  plants  are  apt  to  be  arranged 
according  to  depth,  the  bulrushes  standing  in  the  deepest 
water,   and  behind  them  the  reed  grasses,   and   then  the 


186 


PLANT   RELATIONS. 


cat -tails.     This  regular  arrangement  in  zones  is  so  often 
interfered  with,  however,  that  it  is  not  always  evident. 

The  reed-swamp  societies  have  been  called  "  the  pioneers 
of  land  vegetation,"  for  the  detritus  collects  about  them, 


i  \  $ 


H 


Fig.  It33.  A  reed  swamp,  fringing  the  low  shore  of  a  lake  or  a  sluggish  stream.  The 
plants  are  tall  and  wand-like,  and  all  are  monocotyls.  Three  types  are  prominent, 
the  reed  grasses  (the  tallest),  the  cat-tails  (at  the  right),  and  the  bulrushes  (a  group 
standing  out  in  deeper  water  near  the  middle  of  the  fringing  growth).  The  plant 
in  the  foreground  at  the  extreme  right  is  the  arrow-leaf  {Sagittaria),  recognized 
by  its  characteristic  leaves.— After  Keener. 


the  water  becomes  more  and  more  shallow,  until  finally 
the  reed  plants  are  compelled  to  migrate  into  deeper  water 
(see  §108).  In  this  way  small  lakes  and  ponds  may  be 
completely  reclaimed,  and  become  converted  first  into 
ordinary  swamps,  and  finally  into  wet  meadows.     Instances 


HYDROPHYTE    SOCIETIES.  187 

of  nearly  reclaimed  ponds  may  be  noticed,  where  bulrushes, 
cat-tail  flags,  and  reed  grasses  still  occupy  certain  wet 
spots,  but  are  shut  off  from  further  migration.  The  social 
growth  of  these  plants,  brought  about  by  extensive  root- 
stock  development,  is  especially  favorable  for  detaining 
detritus  and  building  a  land  surface. 

Reed-swamp  plants  also  have  in  general  a  tall  and  un- 
branched  habit  of  body.  They  may  be  bare  and  leafless, 
with  a  terminal  cluster  of  flowers,  as  in  the  bulrushes  ;  or 
the  wand-like  stems  may  bear  long,  linear  leaves,  as  in  the 
cat-tails  ;  or  the  stem  may  be  a  tall  stalk  with  two  rows 
of  narrow  leaves,  as  in  the  reed  grasses.  No  more  charac- 
teristic group  of  forms  is  found  in  any  society.  Of  course, 
associated  with  these  forms  are  also  free  and  fixed  hydro- 
phytes, which  characterize  the  other  societies. 

138.  Swamp-moors. — The  word  moor  is  used  to  designate 
the  meadow-like  expanses  of  swampy  ground.  Here  belong 
the  ordinary  swamps,  marshes,  bogs,  etc.  There  is  less 
water  than  in  the  case  of  the  reed  swamps,  and  often  very 
little  standing  water.  One  of  the  peculiarities  of  the 
swamp-moor  is  that  the  water  is  rich  in  the  soil  materials 
used  in  food  manufacture,  notably  the  nitrates  from  which 
nitrogen  is  obtained  for  proteid  manufacture.  In  such 
conditions,  therefore,  the  vegetation  is  dense,  and  the  soil 
is  black  with  the  humus  derived  from  the  decaying  plant 
bodies. 

Typical  swamp-moors  border  the  reed  swamps  on  the 
land  side,  and  slowly  encroach  upon  them  as  the  reed 
plants  build  up  land.  Probably  the  most  characteristic 
plant  forms  of  the  swamp-moor  arc  the  sedges,  and  asso- 
ciated with  them  are  certain  coarse  grasses.  These  give 
the  meadow-like  aspect  to  the  swamp,  although  these  grass- 
like forms  are  very  coarse.  Along  witli  the  dominant 
sedges  and  grasses  are  numerous  other  plants  adapted  to 
such  conditions,  such  as  some  of  the  buttercups.  It  would 
be  impracticable  to  give  a  list  of  swamp-moor  plants,  as  the 


L88  PLANT    RELATIONS. 

forms  associated  with  sedges  and  grasses  may  vary  widely 
in  different  societies  (Fig.  1G4). 

In  almost  all  swamp-moors  there  is  a  lower  stratum  of 
vegetation  than  that  formed  by  the  sedges.  This  lower 
stratum  is  made  of  certain  swamp  mosses,  which  grow  in 
very  dense  masses.  Towards  the  north,  where  the  tempera- 
ture conditions  are  not  so  favorable  for  the  sedge  stratum, 
it  may  be  lacking  almost  entirely,  and  only  the  lower  moss 
stratum  left.  In  these  cases  the  swamp-moor  becomes 
little  more  than  a  great  bed  of  moss,  and  it  is  in  such  con- 
ditions that  peat  may  be  formed. 

139.  Swamp-thickets. — Swamp-thickets  are  very  closely 
associated  with  swamp-moors,  and  are  doubtless  derived 
from  them.  If  a  swamp-moor,  with  its  sedge  stratum  and 
moss  stratum,  be  invaded  by  shrubs  or  low  trees,  it  becomes 
a  swamp-thicket.  It  will  be  noticed  that  these  shrubs  and 
trees  are  of  very  uniform  type,  being  mainly  willows,  alders, 
birches,  etc.  Such  willow  and  alder  thickets  are  very  com- 
mon in  high  latitudes. 

II.  Xerophytic  hydrophytes. 
A.  Fresh-water  societies. 

140.  Sphagnum-moors. — The  sphagnum-moor  is  a  very 
peculiar  type  of  swamp  society.  It  is  so  named  because  the 
common  bog  or  peat  moss,  known  as  sphagnum,  gives  a 
peculiar  stamp  to  the  whole  area.  Sphagnums  are  large, 
pale  mosses,  whose  lower  parts  may  die,  and  whose  upper 
parts  continue  to  live  and  put  out  new  branches,  so  that  a 
dense  turf  is  formed.  In  walking  over  such  a  bog  the  moss 
turf  seems  springy,  and  sometimes  trembles  so  as  to  sug- 
gest the  name  "quaking  bog."  These  are  the  great  peat- 
forming  bogs.  It  is  interesting  to  know  what  conditions 
keep  the  swamp-moor  plants  out  of  the  sphagnum-moor. 
The  plants  of  the  sphagnum-moor  seem  to  be  entirely  dif- 
ferent from  those  of  the  swamp-moor,  although  the  amount 


HYDROPHYTE    SOCIETIES.  189 

of  water  is  approximately  the  same.  Not  only  are  the 
plants  different  in  the  sphagnum-moor,  but  they  are  not  so 
numerous,  and,  with  the  exception  of  the  moss,  do  not 
grow  so  densely.  It  is  to  be  noticed  that  creeping  plants 
are  abundant,  and  also  many  forms  which  are  known  to 
obtain  their  food  material  already  manufactured,  and  there- 
fore are  saprophytes.  Certain  kinds  of  sedges  and  grasses 
are  found,  but  generally  not  those  of  the  swamp-moor, 
while  heaths  and  orchids  are  especially  abundant.  It  is  in 
these  sphagnum-moors,  also,  that  the  curious  forms  of  car- 
nivorous plants  are  developed,  among  which  the  pitcher 
plants,  droseras,  and  dionasas  have  been  described.  In 
considering  this  strange  collection  of  forms,  it  is  evident 
that  there  must  be  some  peculiarity  in  the  food  supply,  for 
the  heaths  and  orchids  are  notorious  for  their  partial  sap- 
rophytic habits,  and  the  carnivorous  plants  are  so  named 
because  they  capture  insects  to  supplement  their  food  sup- 
ply. The  fact,  also,  that  the  peculiar  sphagnum  mosses, 
rather  than  the  mosses  of  the  swamp-moor,  are  the  prevalent 
ones,  indicates  the  same  thing. 

It  has  been  discovered  that  the  water  of  the  sphagnum- 
moor  is  very  poor  in  the  food  materials  which  are  abundant 
in  the  water  of  the  swamp-moor.  There  is  a  special  lack 
of  the  materials  which  are  used  in  the  manufacture  of  pro- 
teids,  and  hence  this  process  is  seriously  interfered  with. 
It  is  necessary,  therefore,  to  obtain  proteids  already  formed 
in  animals,  or  in  other  plants.  This  will  account  for  the 
necessity  of  the  saprophytic  habit,  and  of  the  carnivorous 
habit,  and  for  the  sphagnum  mosses  which  can  endure  such 
conditions.  Of  course,  it  also  accounts  for  the  exclusion 
of  the  characteristic  plants  of  the  swamp-moor. 

Another  peculiarity  in  connection  with  the  sphagnum- 
moor,  aside  from  its  poverty  in  food  material,  is  the  lack 
of  those  low  plant  forms  (bacteria)  which  induce  decay. 
Bacteria  are  very  minute  plants,  some  of  which  are  active 
agents  in  processes  of  decay,  and  when  these  are  absent 


190  PLANT   KELATIONS. 

decay  is  checked.  As  a  consequence,  the  sphagnum-moor 
waters  are  strongly  antiseptic;  that  is,  they  prevent  decay 
by  excluding  certain  bacteria.  It  is  a  well-known  fact  that 
bodies  of  men  and  animals  which  have  become  submerged 
in  sphagnum-bogs  may  not  decay,  but  have  been  found 
preserved  after  a  very  long  period.  This  will  also  indicate 
why  such  bogs  are  especially  favorable  for  peat  formation. 

These  two  types  of  moors,  therefore,  may  be  contrasted 
as  follows  :  The  swamp-moor  is  rich  in  plant  food,  and  is 
characterized  chiefly  by  grassy  plants  ;  the  sphagnum-moor 
is  poor  in  food  material,  and  is  characterized  chiefly  by 
sphagnum  moss.  It  will  be  noted  that  peat  may  be  formed 
in  connection  with  both  of  these  moors,  but  in  the  swamp- 
moor  the  plant  forms  cannot  be  distinguished  in  the  peat, 
as  they  have  been  more  or  less  disorganized  through  decay, 
while  in  the  peat  of  the  sphagnum-moor  the  plant  forms 
are  well  preserved.  The  peat  of  the  swamp-moor,  also, 
yields  a  great  amount  of  ash,  for  the  swamp-moor  is  rich 
in  soil  materials,  while  the  peat  of  the  sphagnum-moor 
yields  very  little  ash. 

141.  Swamp  forests. — It  was  noted  that  the  special  types 
of  shrub  or  tree  growth  associated  with  the  swamp-moor 
conditions  are  willows,  alders,  birches,  etc.  In  the  same 
way  there  is  a  peculiar  tree  type  associated  with  the 
sphagnum-moor.  It  is  very  common  to  have  a  sphagnum 
area  occupied  by  trees,  and  the  area  becomes  a  swamp 
forest,  rather  than  a  sphagnum-moor.  The  chief  tree 
type  which  occupies  such  conditions  is  the  conifer  type, 
popularly  known  as  the  evergreens.  The  swamp  forests, 
therefore,  with  a  sphagnum-moor  foundation,'  are  made  up 
of  larches,  certain  hemlocks  and  pines,  junipers,  etc.,  and 
towards  the  south  the  cypress  comes  in  (see  Fig.  165). 
The  larch  is  a  very  common  swamp  tree  of  the  northern 
regions,  where  such  an  area  is  commonly  called  a  u  tama- 
rack swamp  "  (see  Fig.  158).  The  larch  forests  are  apt  to 
be  in  the  form  of  small  patches,  while  the  larger  swamp 


o  3 

OD     O 


192 


PLANT   RELATIONS. 


forests  are  made  of  dense  growths  of  hemlocks,  pines,  etc. 
In  the  densest  of  these  forests  the  shade  is  so  complete 
that  there  may  be  very  few  associated  plants  occurring  in 
strata  between  the  sphagnum  moss  and  the  trees.  In  the 
larch  forests,  however,  the  undergrowth  may  be  very  dense. 

B.  Salt-water  societies. 

142.  Mangrove  swamps. — This  is  certainly  the  most  vig- 
orous of  the  salt-water  societies.  Mangrove  swamps  occur 
along  flat  tropical  seacoasts,  where  the  waters  are  quiet. 


angrove  forest  advari 


3  water.— After  Schimfer. 


The  mangrove  is  a  tree  of  curious  habit,  which  advances 
slowly  out  into  the  water  and  extends  back  landwards  as 
low  woods  or  thickets  (see  Figs.  166,  167).  The  whole 
surroundings  appear  forbidding,  for  the  water  is  sluggish 
and  mucky,  covered  with  scum,  rich  in  bacteria,  and  with 
bubbles  constantly  breaking  upon  the  surface  from  decay- 
ing matter  beneath  the  water.     The  mangrove  has  the  pe- 


192b  PLANT   RELATIONS. 

culiarity  of  germinating  its  seeds  while  still  upon  the  tree, 
so  that  embryos  hang  from  the  trees,  and  then  drop  like 
plumb-bobs  into  the  muck  beneath,  where  they  stick  fast 
and  are  immediately  in  a  condition  to  establish  themselves. 
In  these  mangrove  swamps  the  species  are  few,  and  the 
adaptations  chiefly  in  the  way  of  developing  various  kinds 
of  holdfasts  for  anchoring  in  the  uncertain  soil,  and  also 
various  devices  for  carrying  air  to  the  submerged  parts. 

14:>.  Beach  marshes  and  meadows.— The  salt  marshes 
and  meadows  near  the  seacoast  are  very  well  known.  They 
lie  beyond  the  reach  of  ordinary  flood  tide,  but  the  waters 
are  brackish.  In  these  marshes  and  meadows  occur  certain 
characteristic  salt-water  grasses  and  sedges.  Such  forms 
being  the  dominant  type  give  the  general  appearance  of 
a  coarse  meadow.  The  difference  between  a  marsh  and 
meadow  is  simply  a  question  of  the  amount  of  water. 


CHAPTER   XIII. 

XEROPHYTE    SOCIETIES. 

144.  General  character. — Strongly  contrasted  with  the 
hydrophytes  are  the  xerophytes,  which  are  adapted  to  dry 
air  and  soil.  The  xerophytic  conditions  may  he  regarded 
in  general  as  drouth  conditions.  It  is  not  necessary  for 
the  air  and  soil  to  he  dry  throughout  the  year  to  develop 
xerophytic  conditions.  These  conditions  may  be  put  under 
three  heads  :  (1)  possible  drouth,  in  which  a  season  of 
drouth  may  occur  at  irregular  intervals,  or  in  some  seasons 
may  not  occur  at  all ;  (2)  periodic  drouth,  in  which  there 
is  a  drouth  period  as  definite  as  the  winter  period  in  cer- 
tain regions  ;  (3)  perennial  drouth,  in  which  the  dry  con- 
ditions are  constant,  and  the  region  is  distinctly  an  arid 
or  desert  region. 

However  xerophytic  conditions  may  occur,  the  problem 
of  the  plant  is  always  one  of  water  supply,  and  many  strik- 
ing structures  have  been  developed  to  answer  it,  Plants 
in  such  conditions  must  provide,  therefore,  for  two  things: 
(1)  collection  and  retention  of  water,  and  (2)  prevention  of 
its  loss.  It  is  evident  that  in  these  drouth  conditions  the 
loss  of  water  through  transpiration  (see  §20)  tends  to  be 
much  increased.  This  tendency  in  the  presence  of  a  very 
meager  water  supply  is  a  menace  to  the  life  of  the  plant. 
It  is  impracticable  to  stop  transpiration  entirely,  for  it 
must  take  place  in  connection  with  a  necessary  life-process. 
The  adaptations  on  the  part  of  the  plant,  therefore,  are 
directed  towards  the  regulation  of  transpiration,  that  it 


194  PLANT    RELATIONS. 

may  occur  sufficiently  for  the  life-processes,  but  that  it 
may  not  be  wasteful. 

The  regulation  of  transpiration  may  be  accomplished 
in  two  general  ways.  It  will  be  remembered  that  the 
amount  of  transpiration  holds  some  relation  to  the 
amount  of  leaf  exposure  or  exposure  of  green  tissue. 
Therefore,  if  the  amount  of  leaf  exposure  be  diminished, 
the  total  amount  of  transpiration  will  be  reduced.  Another 
general  way  for  regulating  transpiration  is  to  protect 
the  exposed  surface  in  some  way  so  that  the  water  does 
not  escape  so  easily.  In  a  word,  therefore,  the  general 
method  is  to  reduce  the  extent  of  exposed  surface  or  to 
protect  it.  It  must  be  understood  that  plants  do  not  differ 
from  each  other  in  adopting  one  or  the  other  of  these 
methods,  for  both  are  very  commonly  used  by  the  same 
plant. 

Adaptations. 

145.  Complete  desiccation. — Some  plants  have  a  very  re- 
markable power  of  completely  drying  up  during  the  drouth 
period,  and  then  reviving  upon  the  return  of  moisture. 
This  power  is  strikingly  illustrated  among  the  lichens  and 
mosses,  some  of  which  can  become  so  dry  that  they  may  be 
crumbled  into  powder,  but  revive  when  moisture  reaches 
them.  A  group  of  club  mosses,  popularly  known  as  "  res- 
urrection plants,"  illustrates  this  same  power.  The  dried 
up  nest-like  bodies  of  these  plants  are  common  in  the 
markets,  and  when  they  are  placed  in  a  bowl  of  water  they 
expand  and  may  renew  their  activity.  In  such  cases  it  can 
hardly  be  said  that  there  is  any  special  effort  on  the  part  of 
the  plant  to  resist  drouth,  for  it  seems  to  yield  completely 
to  the  dry  conditions  and  loses  its  moisture.  The  power 
of  reviving,  after  being  completely  dried  out,  is  an  offset, 
however,  for  protective  structures. 

146.  Periodic  reduction  of  surface. — In  regions  of  periodic 


XEROPHYTE    SOCIETIES. 


195 


drouth  it  is  very  com- 
mon for  plants  to 
diminish  the  exposed 
surface  in  a  very  de- 
cided way.  In  such 
cases  there  is  what 
may  be  called  a  peri- 
odic surface  decrease. 
For  example,  annual 
plants  remarkably 
diminish  their  ex- 
posed surface  at  the 
period  of  drouth  by 
being  represented 
only  by  well-pro- 
tected seeds.  The 
whole  exposed  sur- 
face of  the  plant, 
root,  stem,  and  leaves, 
has  disappeared,  and 
the  seed  preserves  the 
plant  through  the 
drouth. 

Little  less  remark- 
able is  the  so-called 
geophilous  habit.  In 
this  case  the  whole  of 
the  plant  surface  ex- 
posed to  the  air  dis- 
appears, and  only 
underground  parts, 
such  as  bulbs,  tu- 
bers, etc.,  persist  (see 
Figs.  45,  46,  GG,  G7, 
68,  69,  70,  75,  144, 
168, 169).  At  the  re- 
14 


l^*"N       /~\             ~ 

- 

9        V: 

1 

1 

1 
^\  i 

i 

A  °  * 

^hs 

Pig.  168.  The  bloodroot  (Sanguinaria),  showing 
the  subterranean  rootstock  Bending  leaves  and 
flower  above  the  surface.— After  Atkinson. 


196 


PLANT   RELATIONS. 


spring 


Fig.   169.     The 

beauty      (Claytonia), 
showing  subterranean 

tuber-like  stem  sending  leaf  and  flower-bearinf 
Bteni  above  the  surface.— After  Atkinson. 


turn  of  the  moist  season 
these  underground  parts 
develop  new  exposed 
surfaces.  In  such  cases 
it  may  be  said  that  at 
the  coming  of  the  drouth 
the  plant  seeks  a  sub- 
terranean retreat. 

A  little  less  decrease 
of  exposed  surface  is 
shown  by  the  deciduous 
habit.  It  is  known  that 
certain  trees  and  shrubs, 
whose  bodies  remain 
exposed  to  the  drouth, 
shed  their  leaves  and 
thus  very  greatly  reduce 
the  amount  of  exposure  : 
with  the  return  of  mois- 
ture, new  leaves  are  put 
forth.  It  will  be  re- 
marked, in  this  connec- 
tion, that  the  same 
habits  serve  just  as  well 
to  bridge  over  a  period 
of  cold  as  a  period  of 
drouth,  and  perhaps 
they  are  more  familiar 
in  connection  with  the 
cold  period  than  in  con- 
nection with  the  drouth 
period. 

147.  Temporary  reduc- 
tion of  surface. — While 
the  habits  above  have  to 
do  with  regular  drouth 


XEROPHYTE    SOCIETIES.  197 

periods,  there  are  other  habits  by  which  a  temporary  re- 
duction of  surface  may  be  secured.  For  instance,  at  the 
approach  of  a  period  of  drouth,  it  is  very  easy  to  observe 
certain  leaves  rolling  up  in  various  ways.  As  a  leaf  be- 
comes rolled  up,  it  is  evident  that  its  exposed  surface  is 
reduced.  The  behavior  of  grass  leaves,  under  such  cir- 
cumstances, is  very  easily  noted.  A  comparison  of  the  grass 
blades  upon  a  well-watered  lawn  with  those  upon  a  dried-np 
lawn  will  show  that  in  the  former  case  the  leaves  are  flat, 
and  in  the  latter  more  or  less  rolled  up.  The  same  habit 
is  also  very  easily  observed  in  connection  with  the  larger- 
leaved  mosses,  which  are  very  apt  to  encounter  drouth 
periods. 

148.  Fixed  light  position. — In  general,  when  leaves  have 
reached  maturity,  they  are  unable  to  change  their  position 
in  reference  to  light,  having  obtained  what  is  known  as  a 
fixed  light  position.  During  the  growth  of  the  leaf,  how- 
ever, there  may  be  changes  in  direction  so  that  the  fixed 
light  position  will  depend  upon  the  light  direction  during 
growth.  The  position  finally  attained  is  an  expression  of 
the  attempt  to  secure  sufficient,  but  not  too  much  light 
(see  §13).  The  most  noteworthy  fixed  positions  of  leaves 
are  those  which  have  been  developed  in  intense  light. 
A  very  common  position  in  such  cases  is  the  profile  posi- 
tion, in  which  the  leaf  apex  or  margin  is  directed  upwards, 
and  the  two  surfaces  are  more  freely  exposed  to  the  morn- 
ing and  evening  rays — that  is,  the  rays  of  low  intensity — 
than  to  those  of  midday. 

Illustrations  of  leaves  with  one  edge  directed  upwards 
can  be  obtained  from  the  so-called  compass  plants.  Prob- 
ably most  common  among  these  are  the  rosin-weed  of  the 
prairie  region,  and  the  prickly  lettuce,  which  is  an  intro- 
duced plant  very  common  in  waste  ground  (see  Fig.  170). 
Such  plants  received  their  popular  name  from  the  fact  that 
many  of  the  leaves,  when  edgewise,  point  approximately 
north  and  south,  but  this  direction  is  very  indefinite.      It  is 


198 


PLANT   RELATIONS. 


evident  that  such  a  position  avoids  exposure  of  the  leaf 
surface  to  the  noon  rays,  but  obtains  for  these  same  sur- 
faces the  morning  and  evening  rays.  If  these  plants  are 
developed  in  the  shade,  the   "compass"   habit  does  not 


Fig.  170.  Two  compass  plants.  The  two  figures  to  the  left  represent  the  same  plant 
(SUphiutil)  viewed  from  the  ea6t  and  from  the  south.  The  two  figures  to  the  right 
represent  the  same  relative  positions  of  the  leaves  of  Lactuca. — After  Kerner. 


appear  (see  §15).  The  profile  position  is  a  very  common 
one  for  the  leaves  of  Australian  plants,  a  fact  which  gives 
much  of  the  vegetation  a  peculiar  appearance.  All  these 
positions  are  serviceable  in  diminishing  the  loss  of  water, 
which  would  occur  with  exposure  to  more  intense  light. 
149.  Motile  leaves. — Although  in  most  plants  the  mature 


XEKOPHYTE   SOCIETIES. 


199 


leaves  are  in  a  fixed  position,  there  are  certain  ones  whose 
leaves  are  able  to  perform  movements  according  to  the  need. 
Mention  has  been  made  already  of  such  forms  as  Oxalis 
(see  §14),  whose  leaves  change  their  position  readily  in 
reference  to  light.  Motile  leaves  have  been  developed  most 
extensively  among  the  Legnminosce,  the  family  to  which 


Fig.  171.  Two  twigs  of  a  sensitive  plant.  The  one  to  the  left  shows  the  numerous 
small  leaflets  in  their  expanded  position  ;  the  one  to  the  right  shows  the  greatly 
reduced  surface,  the  leaflets  folded  together,  the  main  leaf  branches  having 
approached  one  another,  and  the  main  leaf-stalk  having  bent  sharply  downwards. 
—After  Strasburger. 


belong  peas,  etc.  In  this  family  are  the  so-called  "sen- 
sitive plants,"  which  have  received  their  popular  name 
from  their  sensitive  response  to  light  as  well  as  to  other 
influences  (see  Fig.  171).  The  acacia  and  mimosa  forms 
are  the  most  notable  sensitive  plants,  and  are  especially 
developed  in  arid  regions.  The  leaves  are  usually  very 
large,  but  are  so  much  branched  that  each  leaf  is  com- 
posed  of  very  numerous  small  leaflets.     Each  leaflet  has 


200 


PLANT   RELATIONS. 


the  power  of  independent  motion,  or  the  whole  leaf  may 
move.  If  there  is  danger  from  exposure  to  drouth,  some 
of  the  leaflets  will  be  observed  to  fold  together  ;  in  case 


Fig.  172.     A  heath  plant  (Erica),  showing  low,  hushy  growth  and  small  leaves. 


the  danger  is  prolonged,  more  leaflets  will  fold  together ; 
and  if  the  danger  j)ersists,  the  surface  of  exposure  will  be 
still  further  reduced,  until  the  whole  plant  may  have  its 
leaves  completely  folded  up.     In  this  way  the  amount  of 


XEROPHYTE    SOCIETIES.  201 

reduction  of  the  exposed  surface  may  be  accurately  regu- 
lated to  suit  the  need  (see  §38). 

150.  Reduced  leaves. — In  regions  that  are  rather  per- 
manently dry,  it  is  observed  that  the  plants  in  general  pro- 
duce smaller  leaves  than  in  other  regions  (see  Fig.  173). 
That  this  holds  a  direct  relation  to  the  dry  conditions  is 


Flu.  173.  Leaves  from  the  common  basswood  (TH'ki),  showing  the  effect  of  environ- 
ment ;  those  at  the  right  being  from  a  tree  growing  in  a  river  bottom  (mesophyte 
conditions)  ;  those  at  the  left  being  from  a  tree  growing  upon  a  dune,  where  it  is 
exposed  to  intense  light,  heat,  cold,  and  wind.  Not  only  are  the  former  larger, 
but  they  are  much  thinner.  The  leaves  from  the  dune  tree  are  strikingly  smaller, 
much  thicker,  and  more  compact.— After  Cowles. 

evident  from  the  fact  that  the  same  plant  often  produces 
smaller  leaves  in  xerophytic  conditions  than  in  moist  con- 
ditions. One  of  the  most  striking  features  of  an  arid 
region  is  the  absence  of  large,  showy  leaves  (see  Fig.  172). 
These  reduced  leaves  are  of  various  forms,  such  as  the 
needle  leaves  of  pines,  or  the  thread-like  leaves  of  certain 
sedges  and  grasses,  or  the  narrow  leaves  with  inrolled 
margins  such  as  is  common  in  many  heath  plants.     The 


202 


PLANT   RELATIONS. 


Fig.  174.     Two  species  of  A chillea  on  different  soils.    The  one  to  the  left  was  grown 
in    drier    conditions    and    shows    an    abundant    development    of    hairs.— After 

SCHIMPEK. 


extreme  of  leaf  reduction  has  been  reached  by  the  cactus 
plants,  whose  leaves,  so  far  as  foliage  is  concerned,  have 
disappeared  entirely,  and    the   leaf  work  is    done   by  the 


XEKOPHYTE    SOCIETIES. 


203 


surface  of  the  globular,  cylindrical,  or  flattened  stems  (see 
§36). 

151.  Hairy  coverings. — A  covering  of  hairs  is  an  effective 
sun  screen,  and  it  is  very  common  to  find  plants  of  xerophyte 
regions  character- 
istically hairy  (see 
§35).  The  hairs 
are  dead  struc- 
tures, and  within 
them  there  is  air. 
This  causes  them 
to  reflect  the  light, 
and  hence  to  ap- 
pear white  or 
nearly  so.  This 
reflection  of  light 
by  the  hairs  dimin- 
ishes the  amount 
which  reaches  the 
working  region  of 
the  plant  (see  Fig. 
174). 

152.  Body  habit. 
— Besides  the  va- 
rious   devices   for 
diminishing    ex- 
posure or  leaf  sur- 
face,    and     hence 
loss    of    water, 
enumerated  above, 
the  whole  habit  of 
the  plant  may  em- 
phasize the  same  purpose.    In  dry  regions  it  is  to  be  observed 
that  dwarf  growths  prevail,  so  that  the  plant  as  a  whole 
does  not  present  such  an  exposure  to  the  dry  air  as  in 
regions  of  greater  moisture  (see  Fig.  175).     Also  the  pros- 


/     \           "*"l 

/     %A\      A' 

U 

11     mv 

H          V  WW 
KMB  i     8*1  ■■Mi 

1    • 

j 

Fig.  175.  Two  plants  of  a  common  scouring  rush  (Eqtii- 
setum),  showing  the  effect  of  environment ;  the  long, 
anbranched  one  having  grown  in  normal  meeophyte 
conditions  ;  the  short,  bushy  branching,  more  slender 
form  having  grown  on  the  dunes  (xerophyte  condi- 
tions).—After  COWLES. 


204  PLANT   RELATIONS. 

trate  or  creeping  habit  is  a  much  less  exposed  one  in  such 
regions  than  the  erect  habit.  In  the  same  manner,  the  very 
characteristic  rosette  habit,  with  its  cluster  of  overlapping 
leaves  close  against  the  ground,  tends  to  diminish  loss  of 
water  through  transpiration. 

One  of  the  most  common  results  of  xerophytic  conditions 
upon  body  habit  is  the  development  of  thorns  and  spiny 


Fig.  176.     Young  plants  of  Euphorbia  splendens,  showing  a  development  of  thorns 
characteristic  of  the  plants  of  dry  regions. 


processes.  As  a  consequence,  the  vegetation  of  dry  regions 
is  characteristically  spiny.  In  many  cases  these  spiny  pro- 
cesses can  be  made  to  develop  into  ordinary  stems  or  leaves 
in  the  presence  of  more  favorable  water  conditions.  It  is 
probable,  therefore,  that  such  structures  represent  reduc- 
tions in  the  growth  of  certain  regions,  caused  by  the  unfavor- 
able conditions.  Incidentally  these  thorns  and  spiny  pro- 
cesses are  probably  of  great  service  as  a  protection  to  plants 
in  regions  where  vegetation  is  peculiarly  exposed  to  the 


XEK0P1IYTE  SOCIETIES. 


205 


ravages  of  animals  (see  §105).      Examine  Figs.  176,  177, 
178,  179,  180,  181. 

153.  Anatomical  adaptations. — It  is  in  connection  with 
the  xerophytes  that  some  of  the  most  striking  anatomical 
adaptations  have  been 
developed.  In  such 
conditions  the  epider- 
mis is  aj^t  to  be  cov- 
ered by  layers  of 
cuticle,  which  are  de- 
veloped by  the  walls 
of  the  epidermal  cells, 
and  being  constantly 
formed  beneath,  the 
cnticle  may  become 
very  thick.  This 
forms  a  very  efficient 
protective  covering. 
and  has  a  tendency  to 
diminish  the  loss  of 
water  (see  §35).  It  is 
also  to  be  observed 
that  among  xerophytes 
there  is  a  strong  de- 
velopment of  palisade 
tissue.  The  working 
cells  of  the  leaves  next 
to  the  exposed  surface 
are  elongated,  and  are 
directed  endwise  to 
the  surface.  In  this  way  only  the  ends  of  the  elongated 
cells  are  exposed,  and  as  such  cells  stand  very  closely  to- 
gether, there  is  no  drying  air  between  them.  In  some 
cases  there  may  be  more  than  one  of  these  palisade  rows 
(see  §32).  It  has  been  observed  that  the  chloroplasts  in 
these  palisade  cells  are  able  to  assume  various  positions  in 


Fig.  177.  Two  plants  of  common  gorse  or  furze 
(JJlex)y  showing  the  effect  of  environment  :  b 
is  a  plant  grown  in  moist  conditions  ;  a  is  a 
plant  grown  in  dry  conditions,  the  leaves  and 
branches  having  been  almost  entirely  developed 
as  thorns.— After  Lothei.ikk. 


206 


PLANT   EELATIONS. 


Fig.  178.  A  branch  of  Cylisus,  showing  the 
reduced  leaves  and  thorny  branches.— After 
Keener. 


regulation  of  transpiration,  but 
storage  of  water,  as  it  is  received  at  rare  inter- 
vals. It  is  very  common  to  find  a  certain  re- 
gion of  the  plant  body  given  over  to  this  work, 
forming  what  is  known  as  water  tissue.  In 
many  leaves  this  water  tissue  may  be  distin- 
guished from  the  ordinary  working  cells  by 
being  a  group  of  colorless  cells  (see  Figs.  183, 
184,  185).  In  plants  of  the  drier  regions  leaves 
may  become  thick  and  fleshy  through  acting 
as  water  reservoirs,  as  in  the  case  of  the  agave, 
sedums,  etc.  Fleshy  or  "  succulent "  leaves 
are  regarded  as  adaptations  of    prime  impor- 


the  cell,  so  that  when 
the  light  is  very  intense 
they  move  to  the  more 
shaded  depths  of  the 
cell,  and  when  it  be- 
comes less  intense  they 
move  to  the  more  exter- 
nal regions  of  the  cell 
(see  Fig.  182).  The 
stomata,  or  breathing 
pores,  which  are  devel- 
oped in  the  epidermis, 
are  also  great  regulators 
of  transpiration,  as  has 
been  mentioned  already 
(see  §31). 

154.  Water  reservoirs. 
— In    xero- 
phytes    at-  / 

tention 
must     be 
given     not        i 
only  to  the 
also   to   the 


Fig.  179.  A 
leaf  of  traga- 
canth,  show- 
ing the  re- 
duced leaf- 
lets and  the 
thorn  -like 
tip.— After 
Keener. 


XEROPHYTE   SOCIETIES. 


207 


tance  in  xerophytic  conditions.  In 
the  cactus  plants  the  peculiar  stems 
have  become  great  reservoirs  of 
moisture.  The  globular  body  may 
be  taken  to  represent  the  most  com- 
plete answer  to  this  general  problem, 
as  it  is  the  form  of  body  by  which 
the  least  amount  of  surface  may  be 
exposed  and  the  greatest  amount  of 
water  storage  secured.  In  the  case 
of  fleshy  leaves  and  fleshy  bodies  it 
has  long  been  noticed  that  they  not 
only  contain  water,  but  also  have  a 
great  power  of  re- 


Fig.  181.  Twig  of  com- 
mon locust,  showing 
the  thorns.— After 
Kerner. 


Fig.  180.  A  fragment  of  bar- 
berry, showing  the  thorns. 
— After  Kerner. 

taining  it,     Plant 

collectors  have  found  great  difficulty  in 
drying  these  fleshy  forms,  some  of  which 
seem  to  be  able  to  retain  their  moisture  in- 
definitely, even  in  the  driest  conditions. 
155.  Xerophytic  structure. — The  adap- 
tations given  above  are  generally  found 
in  plants  growing  in  drouth  conditions, 
and  they  all  imply  an  effort  to  diminish 
transpiration.  It  must  not  be  supposed, 
however,  that  only  plants  living  in 
drouth  conditions  show  these  adapta- 
tions. Such  adaptations  result  in  what 
is  known  as  the  xerophytic  structure, 
and  such  a  structure  may  appear  even 
in  plants  growing  in  hydrophyte  condi- 
tions. For  example,  the  bulrush  grows 
in  shallow  water,  and  is  a  prominent 
member  of  one  of  the  hydrophyte  socie- 
ties (see  §137) ;  and  yet  it  has  a  remark- 
ably xerophytic  structure.  This  is  prob- 
ably  due  to  the  fact   that   although   it 


208 


PLANT   RELATIONS. 


stands  in  the  water  its  stem  is  exposed 
to  a  heat  which  is  often  intense. 

The  ordinary  prairie  (see  §169)  is 
included  among  mesophyte  societies 
on  account  of  the  rich,  well-watered 
soil;  and  yet  many  of  the  plants  are 
very  xerophytic  in  structure,  probably 
on  account  of  the  prevailing  dry  winds. 

The  ordinary  sphagnum-bog  (see 
§140),  or  "  peat-bog,"  is  included 
among  hydrophyte  societies.  It  has 
an  abundance  of  water,  and  is  not  ex- 
posed to  blazing  heat,  as  in  the  case 
of  the  bulrushes,  or  to  drying  wind, 
as  in  the  case  of  prairie  plants  ;  and 
yet  its  plants  show  a  xerophytic  struc- 
ture. This  is  found  to  be  due,  proba- 
bly, to  a  lack  of  certain  important  soil 
materials. 

It  is  evident,  therefore,  that  xero- 
phytic structures  are  not  necessarily 
confined  to  xerophytic  situations.  It 
is  probably  true  that  all  societies  which 
show  xerophytic  structures  belong  to- 
gether more  naturally 

_^ 


Fig.  182.  Cells  from  the  leaf 
of  a  quilhvort  (Isoetes). 
The  light  is  striking  the 
cells  from  the  direction  of 
one  looking  at  the  illus- 
tration. If  it  be  some- 
what diffuse  the  chloro- 
plasts  distribute  them- 
selves through  the  shal- 
low cell,  as  in  the  cell  to 
the  left.  If  the  light  be 
intense,  the  chloroplasts 
move  to  the  wall  and  as- 
sume positions  less  ex- 
posed, as  in  the  cell  to 
the  right. 


than  do  the  societies 
which  are  grouped  ac- 
cording to  the  water 
supply. 

Societies. 

No  attempt  will  be 
made  to  classify  these 
very  numerous  socie- 
ties, but  a  few  prom- 


COS 


Fig.  183.  A  section  through  a  Begonia  leaf,  show- 
ing the  epidermis  (ep)  above  and  below,  the 
water-storage  tissue  (ws)  above  and  below,  and 
the  central  chlorophyll  region  (as). 


XEROPHYTE   SOCIETIES. 


209 


Fig.  184.  A  section  through  a  fleshy  leaf  (Clinia),  show- 
ing the  chlorophyll  region  on  the  outside  (shaded  and 
marked  as),  and  the  large  interior  water-storage  region 
(ws). 


inent  illustra- 
tions will  be 
given. 

15G.  Rock 
societies.  —  Vari- 
ous plants  are 
able  to  live  up- 
on exposed  rock 
surfaces,  and 
therefore  form 
distinct  associa- 
tions of  xero- 
phytes.  In  gen- 
eral they  are  lichens,  mosses,  and  crevice  plants  (see  Fig. 
186).  The  crevice  plants  are  those 
which  send  their  roots  into  the  rock 
crevices  and  so  gain  a  foothold. 
The  crevice  plants  also  commonly 
show  a  rosette  habit,  the  rosette  of 
overlapping  leaves  being  against  the 
rock  face,  and  therefore  in  the  most 
favorable  position  for  checking  loss 
of  water. 

157.  Sand  societies. — In  general 
sand  societies  may  be  roughly  grouped 
as  beach  societies,  dune  societies,  and 
sanely  field  societies.  These  three 
hold  a  certain  definite  relation  to 
one  another.  This  natural  relation- 
ship appears  on  the  borders  of  the 
large  lakes,  and  on  seacoasts.  The 
beach  is  nearest  the  water,  the  dunes 
are  next,  and  behind  them  stretch 
the  sandy  fields.  When  the  three 
types  are  thus  associated,  the  plants 
of  the  different  areas  pass  gradually 


Fig.  185.  A  section  through 
a  leaf  of  an  epiphyte, 
showing  a  very  large  de- 
velopment of  water  tissue 
between  the  upper  epi- 
dermis and  the  chloro- 
phyll region,  which  is 
restricted  to  near  the 
under  surface  of  the  leaf. 
—After  Schimper. 


210 


PLANT   RELATIONS. 


into  one  another.  It  is  very  common  to  find  tho  dunes 
omitted  in  the  series,  and  to  have  the  beaches  pass  gradu- 
ally into  the  sandy  fields. 

The  beach  society  is  usually  quite  characteristic,  and  in 
general  it  is  a  poor  flora,  the  beach  being  characteristically 
bare.  The  plants  which  grow  in  such  conditions  are  apt  to 
occur  in  tufts,  or  are  creeping  plants.     It  is  evident  that 


Fig.  186.    A  rock  covered  with  lichens. 


while  the  water  may  seem  to  be  abundant,  it  disappears 
quickly,  so  that  plants  must  adapt  themselves  to  a  dry 
condition  of  the  soil,  which  is  poor  and  with  little  or  no 
accumulation  of  humus.  At  the  same  time,  the  exposure 
to  intense  light  is  extreme.  This  combination  results  in  a 
poor  display  of  individuals  and  of  species.  Here  and  there 
along  beaches,  where  special  conditions  have  favored  the 
accumulation  of  humus,  dense  vegetation  may  spring  up, 
but  it  should  not  be  confused  with  the  ordinary  beach  type. 


15 


212 


PLANT   RELATIONS. 


The  dune  societies  are  subjected  to  very  peculiar  con- 
ditions. Dunes  are  billows  of  sand  that  have  been  devel- 
oped by  prevailing  winds,  and  in  many  cases  they  are  con- 
tinually changing  their  form  and  are  frequently  moving 


-JK  -'£ 


Fig.  188.  A  sandy  field  type,  showing  the  development  of  vegetation  upon  an  old 
beach.  The  vegetation  is  low,  often  tufted  and  heath-like,  being  composed  chiefly 
of  grasses,  beat-berry  (Arctostaphylos)  and  Hudsonia.  In  the  background  to 
the  right  is  a  conifer  forest,  and  between  it  and  the  old  beach  is  seen  a  dense  mass 
of  bearberry,  a  very  characteristic  heath  plant,  and  forming  here  what  is  called  a 
transition  zone  between  the  beach  and  the  forest.— After  Cowles. 


landward  (see  Fig.  187).  The  moving  dunes  should  be 
distinguished  from  the  fixed  ones,  where  the  billow  form  is 
retained,  but  the  dunes  have  ceased  their  motion.  In  the 
case  of  the  active  dunes  a  peculiar  type  of  vegetation  is  de- 
manded.    As  is  to  be  expected,  the  flora  is  very  scanty,  and 


214  PLANT   RELATIONS. 

has  two  remarkably  developed  characters.  The  plants  are 
what  are  known  as  "sand-binders/'  that  is,  the  underground 
structures  become  extremely  developed,  reaching  to  great 
distances  horizontally  and  vertically,  so  that  one  is  always 
surprised  at  the  extent  of  the  underground  system.  This 
wide  searching  for  water  results  in  giving  the  plants  a  deep 
anchorage  in  the  shifting  soil,  and  at  the  same  time  helps 
to  prevent  the  shifting.  As  soon  as  enough  of  the  sand- 
binders  have  established  themselves,  a  shifting  dune  becomes 
a  fixed  one.  Another  characteristic  that  must  be  strongly 
developed  by  these  plants  is  the  ability  to  grow  up  through 
the  sand  after  they  have  been  engulfed.  The  plants  of  the 
shifting  dunes  are_  often  buried  as  the  dune  shifts,  and 
unless  the  burial  has  been  too  deep,  they  are  able  to  continue 
their  development  until  leaves  may  be  exposed  to  the  air. 
In  this  way  plants  have  often  developed  a  length  of  stem 
which  is  far  beyond  anything  they  attain  when  growing  in 
ordinary  conditions. 

The  sandy  field  societies  are  represented  by  a  much 
more  abundant  flora  than  the  beach  or  the  dune  societies, 
the  general  character  being  tufted  grasses  and  low  shrubby 
growths  (see  Fig.  188). 

158.  Shrubby  heaths. — The  shrubby  heaths  are  very 
characteristic  of  the  more  northern  regions,  and  are  closely 
related  to  the  sandy  field  societies.  The  heath  soil  is  apt 
to  be  a  mixture  of  coarse  sand,  or  gravel  and  rock,  with 
an  occasional  deposit  of  humus,  and  would  be  regarded 
in  general  as  a  sterile  soil.  The  flora  of  the  shrubby 
heaths  shows  well-marked  strata,  the  upper  one  being  the 
low  shrubby  plants  of  the  heath  family,  most  prominent 
among  which  are  huckleberries  and  bearberries  (see  Fig. 
172).  The  lower  stratum  is  made  up  of  mosses  and  li- 
chens. A  branching  lichen,  usually  spoken  of  as  the 
"reindeer  moss/'  often  occurs  in  immense  patches  on 
such  heaths.  While  these  shrubby  heaths  occur  most 
extensively  towards   the   north,   small  areas  showing  the 


Nil 


216  PLANT    RELATIONS. 

same  general  character  are  common  in  almost  all  temper- 
ate regions. 

159.  Plains. — Under  tins  head  are  included  great  areas 
in  the  interior  of  continents,  where  dry  air  and  wind 
prevail.  The  plains  of  the  United  States  extend  from 
about  the  one  hundredth  meridian  westward  to  the  foot- 
hills of  the  Rocky  Mountains.  Similar  great  areas  are 
represented  by  the  steppes  of  Siberia,  and  in  the  interior  of 
all  continents.  These  regions  have  been  regarded  as  semi- 
desert  areas,  but  they  are  found  for  the  most  part  to  be 
far  from  the  real  desert  conditions.  They  are  certainly 
areas  of  comparative  dryness,  on  account  of  the  dry  winds 
which  prevail. 

Taking  the  plains  of  the  United  States  as  a  type,  a  very 
characteristic  plant  physiognomy  is  presented  (see  Fig. 
189).  In  general,  there  is  a  meadow-like  expanse,  but  the 
vegetation  is  much  more  sparse  than  in  meadows,  and  is 
much  more  dense  than  in  deserts.  The  two  characteristic 
plant  forms  are  the  bunch  grasses,  that  is,  grasses  which 
grow  in  great  tufts  ;  and  low  grayish  shrubs,  predomi- 
nantly "  sage  brush."  Under  the  shelter  of  the  sage  brush 
or  other  bush  forms,  many  low  herbs  succeed  in  growing. 
In  such  areas  the  growing  season  is  very  short,  during 
which  time  the  vegetation  looks  vigorous  and  fresh  ;  but 
during  the  rest  of  the  year  it  is  very  dull.  In  some  parts 
the  plain  is  dry  enough  to  permit  the  growth  of  the  prickly- 
pear  cactus  (Opuntia),  which  may  take  possession  of  ex- 
tensive areas  (see  Fig.  190). 

Usually  there  are  two  rest  periods  during  the  year, 
developed  by  the  summer  drouth  and  the  winter  cold.  As 
a  consequence,  the  plants  of  the  area  are  partly  spring 
plants,  which  are  apt  to  be  very  brilliant  in  flower  ;  and 
partly  the  later,  deep-rooted  forms.  Over  such  areas  the 
transportation  of  seeds  by  the  wind  is  very  prominent,  as 
the  force  of  the  wind  and  the  freedom  of  its  sweep  make 
possible  very  wide  distribution.     It  is  in  such  areas  that 


^fj>-m,^s  E*je  •  t  ?  **■'  iff*!,*'  '.-x-  r»'     - 


Fig.  192.  Two  plants  of  the  giant  cactus.  Note  the  fluted,  clumsy  branching,  leaf- 
less bodies  growing  from  the  rocky,  sterile  soil  characteristic  of  cactus  deserts. 
Certain  dry-ground  grasses  and  low,  shrubby  plants  with  small  leaves  may  be  seen 
in  the  foreground. 


220 


PLANT   RELATIONS. 


the  tumbleweecl  habit  is  prominently  developed.     Certain 
low  and   densely  branching  plants  are  lightly  rooted  in 


the  soil,  so  that  at  the  close  of  their  gro1 


period  they 


are  easily  uprooted  by  the  wind,  and  are  rolled  to  great 


Fig.  194.    Tree-like  yuccas  from  the  arid  regions  of  Africa,  showing  the  very  mimer 
ous  thick  and  pointed,  sword-like  leaves. 


222  PLANT   KELATIONS. 

distances.  Where  some  barrier,  such  as  a  fence,  lies  across 
the  track  of  the  wind,  these  tumbleweeds  may  accumulate 
in  great  masses.  This  tumbling  over  the  surface  results 
in  an  extensive  scattering  of  seeds  (see  Fig.  120). 

The  prairies,  so  characteristic  of  the  United  States,  are 
regarded  by  some  as  belonging  to  the  plains.  They  cer- 
tainly are  closely  related  to  them  in  origin,  but  can  hardly 
be  regarded  as  being  included  in  xerophyte  conditions,  as 
the  conditions  of  water  supply  and  soil  are  characteristically 
mesophyte,  under  which  head  they  will  be  considered. 

160.  Cactus  deserts. — In  passing  southward  on  the 
plains  of  the  United  States,  it  is  to  be  noted  that  the  con- 
ditions become  more  and  more  xerojmytic,  and  that  the 
bunch  grasses  and  sage  brush,  peculiar  to  the  true  plains, 
gradually  merge  into  the  cactus  desert,  which  represents 
a  region  wdiose  conditions  are  intermediate  between  true 
plains  and  true  deserts  (see  Fig.  191).  In  the  United  States 
this  characteristic  desert  region  begins  to  appear  in  West- 
ern Texas,  New  Mexico,  Arizona,  and  Southern  California, 
and  stretches  far  down  into  the  Mexican  possessions.  This 
vast  arid  region  has  developed  a  peculiar  flora,  which  con- 
tains most  highly  specialized  xerophytic  forms.  The  va- 
rious cactus  forms  may  be  taken  as  most  characteristic, 
and  associated  with  them  are  the  agaves  and  the  yuccas. 
Not  only  are  the  adaptations  for  checking  transpiration 
and  for  retaining  water  of  the  most  extreme  kind,  but 
there  is  also  developed  a  remarkable  armature.  It  is  evi- 
dent that  such  succulent  bodies  as  these  plants  present 
might  speedily  disappear  through  the  attacks  of  animals, 
were  it  not  for  the  armor  of  spines  and  bristles  and  rigid 
walls.     Study  Figs.  38,  39,  40,  192,  193,  194. 

161.  Tropical  deserts. — In  such  areas  xerophyte  con- 
ditions reach  the  greatest  extreme  in  the  combination  of 
maximum  heat  and  minimum  water  supply.  It  is  evident 
that  such  a  combination  is  almost  too  difficult  for  plants 
to  endure.     That  the  very  scanty  vegetation  is  due  to  lack 


224  PLANT   RELATIONS. 

of  water,  and  not  to  lack  of  proper  materials  in  the  soil,  is 
shown  by  the  fact  that  where  water  does  occur  oases  are 
developed,  in  which  luxuriant  vegetation  is  found. 

The  desert  which  extends  from  Egypt  across  Arabia  may 
be  regarded  as  a  typical  one.  It  is  to  be  noted  that  the 
vegetation  is  so  scanty  that  the  soil  is  the  conspicuous 
feature,  and  really  gives  the  characteristic  physiognomy 
(see  Fig.  196).  Accordingly  the  appearance  of  the  deserts 
will  depend  upon  whether  the  desert  soil  is  rocky,  or  of 
small  stones,  or  gravel  (as  in  the  Desert  of  Sahara),  or  of 
red  clay,  or  of  the  dune  type.  As  is  to  be  expected,  such 
vegetation  as  does  occur  is  of  the  tuft  and  bunch  type,  as 
developed  by  certain  grasses,  or  of  the  low  irregular  bush 
type  (see  Fig.  195). 

In  the  South  African  deserts  certain  remarkable  plants 
have  been  noted  which  have  attained  a  certain  amount  of 
protection  through  mimicry,  rather  than  by  means  of  armor, 
as  in  the  case  of  the  cactus  forms.  Some  of  these  plants 
resemble  the  ordinary  stones  lying  about  upon  the  desert. 
With  the  tropical  deserts  should  not  be  confused  such 
areas  as  those  about  the  Dead  Sea,  or  in  the  Death's  Valley 
in  Southern  California,  as  the  barrenness  of  these  areas  is 
due  to  the  strongly  alkaline  soils,  and  therefore  they  belong 
to  the  halophyte  areas. 

162.  Thickets. — The  xerophyte  thicket  is  the  most 
strongly  developed  of  all  thicket  growths.  Mention  has 
been  made  of  willow  and  alder  thickets  in  hydrophyte  con- 
ditions, but  these  are  not  to  be  compared  in  real  thicket 
characters  with  the  xerophyte  thickets.  These  thickets 
are  especially  developed  in  the  tropics  and  subtropics,  and 
may  be  described  as  growths  which  are  scraggy,  thorny, 
and  impenetrable.  Warming  speaks  of  these  thickets  as 
"the  unsuccessful  attempt  of  Nature  to  form  a  forest." 
Evidently  the  conditions  are  not  quite  favorable  for  for- 
est development,  and  an  extensive  thicket  is  the  result. 
Such  thickets  are  well  developed  in  Texas,  where  they  are 


Fig.  198.  A  xerophyte  conifer  forest  in  the  mountains.  The  peculiar  conifer  habit 
of  body  is  recognized,  the  trees  finding  foothold  in  the  crevices  of  rocks  or  in 
areas  of  rock  debris. 


XEROPHYTE   SOCIETIES.  227 

spoken  of  as  "  chaparral."  These  chaparrals  are  notably 
composed  of  mesquit  bushes,  acacias  and  mimosas  of  vari- 
ous sorts,  and  other  plants.  Similar  thickets  in  Africa  and 
Australia  are  frequently  spoken  of  as  "  bush  "  or  "  scrub." 
In  all  of  these  cases  the  thicket  has  the  same  general  type, 
and  probably  represents  one  of  the  most  forbidding  areas 
for  travel. 

163.  Forests.— The  xerophyte  forest  societies  may  be 
roughly  characterized  under  three  general  heads  : 

(1)  Coniferous  forests. — These  forests  are  very  common 
in  xerophyte  conditions  to  the  north,  and  also  in  the  more 
sterile  regions  towards  the  south  (see  Figs.  198  to  201). 
They  are  generally  spoken  of  as  evergreen  forests,  although 
the  name  is  not  distinctive.  These  forests  are  of  several 
types,  such  as  true  pine  forests,  in  which  pines  are  the 
prevailing  trees  and  the  shade  is  not  dense  ;  the  fir  and 
hemlock  forests,  which  are  relatively  dark  ;  and  the  mixed 
forests,  in  which  there  is  a  mingling  of  various  conifers. 
In  such  forests  the  soil  is  often  very  bare,  and  such  under- 
growth as  does  occur  is  largely  composed  of  perennial 
plants.  Many  characteristic  shrubs  with  fleshy  fruits  oc- 
cur, such  as  huckleberries,  bearberries,  junipers,  etc.  It 
will  be  noted  that  in  these  forests  a  characteristic  adapta- 
tion to  xerophyte  conditions  is  the  development  of  needle 
leaves,  which  are  not  only  narrow,  thus  presenting  a  small 
exposure  of  surface,  but  also  have  heavy  walls,  which 
further  prevents  excessive  transpiration. 

(2)  Foliage  forests. — These  are  more  characteristic  of 
tropical  and  subtropical  xerophyte  regions.  Illustrations 
may  be  obtained  from  the  eucalyptus,  a  characteristic 
Australian  forest  tree,  the  live  oaks,  oleanders,  etc  It 
will  be  noticed  that  in  these  cases  the  leaves  are  not  so 
narrow  as  the  needles  of  conifers,  but  are  generally  lance- 
shaped,  and  stiff  and  leathery,  indicating  heavy  walls  to 
reduce  transpiration. 

(3)  Leaf  ess  forests. — In  Java  and  other  oriental  regions 

10 


i&feft. 


Fia,  199. — A  xerophyte  conifer  forest  in  the  Cumberland    Mountains    of    Tennessee. 

The  table  mountain  pines  find  footholds  in  crevices  of  the  rocks. 


Fig.  200.     A  pine  forest,  showing  the  slender,  tall,  continuous  trunks  and  compara- 
tively little  undergrowth.— After  Schimpbk. 


XEKOPHYTE   SOCIETIES.  231 

areas  of  dry  naked  soil  are  sometimes  occupied  by  forest 
growths  which  show  no  development  of  leaves,  the  tree-like 
forms  appearing  continually  bare.  The  oriental  leafless 
tree  form  is  mostly  a  Gasuarina.  Bordering  the  Gulf  of 
California,  both  in  Mexico  proper  and  in  Lower  California, 
there  are  leafless  forests  composed  of  various  kinds  of  giant 
cactus  (see  Fig.  192),  known  as  the  "  cardon  forests."  These 
leafless  forests  represent  the  most  extreme  xerophyte  condi- 
tions occupied  by  plant  forms  which  may  be  regarded  as 
trees. 

164.  Salt  steppes. — In  addition  to  the  xerophyte  socie- 
ties enumerated  above,  in  which  the  water  though  scanty 
is  fresh,  the  two  following  may  be  considered.  The  soil 
and  air  are  relatively  dry,  as  in  ordinary  xerophytic  condi- 
tions, but  the  water  is  more  or  less  saturated  with  common 
salt  or  alkaline  salts.  The  salt  steppes  are  interior  arid 
wastes,  which  probably  mark  the  position  of  old  sea  basins. 
In  the  United  States  one  of  the  most  extensive  of  the  salt 
steppes  is  in  the  Great  Salt  Lake  basin  (see  Fig.  202).  It 
is  here  that  members  of  the  chenopod  family  are  especially 
at  home,  such  as  greasewoods,  seablights,  samphires,  etc., 
for  more  than  any  other  plants  have  they  learned  to  endure 
such  extremely  unfavorable  conditions.  An  extensive  alka- 
line waste  in  the  United  States  is  that  known  as  the  Bad 
Lands,  which  stretches  over  certain  portions  of  Nebraska 
and  Dakota,  and  in  which  the  waters  are  strongly  alkaline. 

165.  Salt  and  alkaline  deserts. — In  these  areas  the  water 
supply  reaches  its  minimum,  and  therefore  the  water  be- 
comes saturated  with  the  characteristic  salts  of  the  soil. 
Xo  worse  combination  for  plant  activity  can  be  imagined 
than  the  combination  of  minimum  water  and  maximum 
salts.  In  consequence,  such  areas  are  almost,  if  not  abso- 
lutely, devoid  of  vegetation.  As  illustrations,  the  exten- 
sive desert  of  the  Dead  Sea  region  and  the  Death's  Valley 
in  Southern  California  may  be  cited. 


c   tc 


>  1 

» 


go     QJ 


CHAPTER  XIV. 

MESOPHYTE    SOCIETIES. 

166.  General  characters. — Mesophytes  make  up  tne  com- 
mon vegetation  of  temperate  regions,  the  vegetation  most 
commonly  met  and  studied.  The  conditions  of  moisture 
are  medium,  precipitation  is  in  general  evenly  distributed, 
and  the  soil  is  rich  in  humus.  The  conditions  are  not  ex- 
treme, and  therefore  special  adaptations,  such  as  are  neces- 
sary for  xerophyte  or  hydrophyte  conditions,  do  not  appear. 
This  may  be  regarded  as  the  normal  plant  condition.  It 
is  certainly  the  arable  condition,  and  most  adapted  to  the 
plants  which  men  seek  to  cultivate.  When  for  purposes 
of  cultivation  xerophyte  areas  are  irrigated,  or  hydrophyte 
areas  are  drained,  it  is  simply  to  bring  them  into  mesophyte 
conditions. 

In  looking  over  a  mesophyte  area  and  contrasting  it 
with  a  xerophyte  area,  one  of  the  first  things  evident  is  that 
the  former  is  far  richer  in  leaf  forms.  It  is  in  the  meso- 
phyte conditions  that  foliage  leaves  show  their  remarkable 
diversity.  In  hydrophyte  and  xerophyte  areas  they  are  apt 
to  be  more  or  less  monotonous  in  form.  xVnother  contrast 
is  found  in  the  dense  growth  over  mesophyte  areas,  much 
more  so  than  in  xerophyte  regions,  and  even  more  dense 
than  in  hydrophyte  areas. 

Among  the  mesophyte  societies  must  be  included  not 
merely  the  natural  ones,  but  those  new  societies  which 
have  been  formed  under  the  influence  of  man.  and  which 
do  not  appear  among  xerophyte  and  hydrophyte  societies. 


Fig.  203.     Alpine  vegetation,  showing  the  low  stature,  dense  growth,  and  conspicu- 
ous flowers.— After  Kerner. 


t> 


MESOPIIYTE   SOCIETIES.  235 

These  new  societies  have  been  formed  by  the  introduction 
of  weeds  and  culture  plants. 

167.  The  two  groups  of  societies. — Two  very  prominent 
types  of  societies  are  included  here  under  the  mesophytes, 
although  they  are  probably  as  distinct  from  one  another  as 
are  the  mesophyte  and  xerophyte  societies.  One  group  is 
composed  of  low  vegetation,  notably  the  common  grasses 
and  herbs  ;  the  other  is  a  higher  woody  vegetation,  com- 
posed of  shrubs  and  trees.  The  most  characteristic  types 
under  each  one  of  these  divisions  are  noted  as  follows. 

A.   Grass  and  herb  societies. 

It  should  not  be  inferred  from  this  title  that  most 
grasses  are  not  herbs,  but  it  is  convenient  to  consider 
grasses  and  ordinary  herb  forms  separately. 

108.  Arctic  and  alpine  carpets. — These  are  dense  mats  of 
low  vegetation  occurring  beyond  forest  growth  in  arctic 
regions,  and  above  the  tree  limit  in  high  mountains.  These 
carpet-like  growths  are  a  notable  feature  of  such  regions. 
In  such  positions  the  growing  season  is  very  short,  and  the 
temperature  is  quite  low  at  times,  especially  at  night.  It 
is  evident,  therefore,  that  there  must  be  provision  for  rapid 
growth,  and  also  for  preventing  dangerous  radiation  of 
heat,  which  might  chill  the  active  plant  below  the  point  of 
safety.  It  is  further  evident  that  the  short  season  and  the 
low  temperature  form  a  combination  which  prevents  the 
growth  of  trees  or  shrubs,  or  even  tall  herbs,  because  the 
season  is  too  short  for  them  to  reach  a  protected  condition, 
and  their  more  exposed  young  structures  are  not  in  a  posi- 
tion to  withstand  the  daily  fall  of  temperature. 

These  carpets  of  vegetation  are  notably  fresh-looking, 
indicating  rapid  growth  ;  green,  indicating  an  abundance 
of  chlorophyll  and  great  activity;  thick,  as  they  arc 
mostly  perennials,  developed  from  abundant  underground 
structures  ;  low,  on   account  of  the  short  season  and  low 


v„-   «04      Two  plants  of  a  rock-rose  aMianthemum),  ehowing  the  effect  of  low 
P,G-^ld  »d  2Z  conditio,,.    The  -„ nd  plan.  »  >.,- -  =  •»£ 

S5S55SSSSSSastt 

ground  form.— After  Bonnier. 


MESOPIIYTE   SOCIETIES.  237 

temperature ;  and  soft,  the  low  stature  and  short  life  not 
involving  the  development  of  specially  rigid  structures  for 
support  or  resistance.  In  such  conditions,  as  would  be 
expected,  annuals  are  in  the  minority,  the  plants  being 
mostly  perennial  and  geophilous.  Geophilous  plants  are 
those  which  have  the  habit  of  disappearing  underground 
when  protection  is  needed.  This  is  probably  the  best  adap- 
tation for  total  disappearance  from  the  surface  and  for  rapid 
reappearance  (see  §146).  In  such  conditions,  also,  rosette 
forms  are  very  common,  the  overlapping  leaves  of  the  rosette 
closely  pressed  to  the  ground  diminishing  the  loss  of  heat 
by  radiation.  It  has  also  been  noticed  that  these  arctic  and 
alpine  carpets  show  intense  color  in  their  flowers,  and  often 
a  remarkable  size  of  flower  in  proportion  to  the  rest  of  the 
plant.  Wherever  the  area  is  relatively  moist,  the  carpet  is 
prevailingly  a  grass  mat ;  in  the  drier  and  sandier  spots  the 
herbs  predominate  (see  Fig.  203). 

In  the  case  of  plants  which  can  grow  both  in  the  low 
ground  and  in  the  alpine  region,  a  remarkable  adaptation 
of  the  plant  body  to  the  different  conditions  may  be  noted. 
The  difference  in  appearance  is  sometimes  so  great  that  it 
is  hard  to  realize  that  the  two  plants  belong  to  the  same 
species  (see  Fig.  204). 

169.  Meadows. — This  term  must  be  restricted  to  natural 
meadow  areas,  and  should  not  be  confused  with  those  arti- 
ficial areas  under  the  control  of  man,  which  are  commonly 
mowed.  The  appearance  of  such  an  area  hardly  needs  defi- 
nition, as  it  is  a  well-known  mixture  of  grasses  and  flower- 
ing herbs,  the  former  usually  being  the  predominant  type. 
Such  meadow-like  expanses  are  common  in  connection  with 
forest  areas  (see  Fig.  205),  but  they  are  most  character- 
istically developed  on  flood-plains  along  streams.  In  most 
cases  the  local  meadow  is  probably  an  ephemeral  society,  to 
be  replaced  by  forest  growth. 

The  greatest  meadows  of  the  United  States  are  the  well- 
known  prairies,  which  extend  from  the  Missouri  eastward 


MESOPIIYTE   SOCIETIES.  239 

to  the  forest  regions  of  Illinois  and  Indiana  (see  Fig.  20G). 
The  prairie  is  regarded  by  some  as  a  xerophyte  area,  and  this 
is  a  natural  conclusion  when  one  examines  only  the  struc- 
tures of  the  plants  which  occupy  it.  It  is  certainly  a  tran- 
sition area  between  the  plains  of  the  West  and  the  true 
mesophytic  areas  of  the  East,  and  there  is  a  general  tran- 
sition from  the  more  xerophytic  western  prairies  to  the 
more  mesophytic  eastern  prairies.  Moreover,  in  the  east- 
ern part  of  the  prairie  region  there  is  locally  every  grada- 
tion between  the  strongly  mesophytic  type  of  the  low  ground 
to  the  more  xerophytic  type  of  the  high  ground. 

The  vegetation  of  the  prairies  in  general  is  composed 
of  tufted  grasses  and  perennial  flowering  herbs.  Unfortu- 
nately, most  of  the  natural  prairie  has  disappeared,  to  be 
replaced  by  farms,  and  the  characteristic  prairie  forms  are 
not  easily  seen.  The  flowering  herbs  are  often  very  tall  and 
coarse,  but  with  brilliant  flowers,  such  as  species  of  aster, 
goldenrod,  rosin-weed,  indigo  plant,  lupine,  bush  clover,  etc. 
The  most  characteristic  of  these  forms  show  their  xero- 
phytic adaptations  by  their  rigidity  and  roughness. 

The  origin  of  the  prairie  has  long  been  a  vexed  question, 
which  has  usually  taken  the  form  of  an  inquiry  into  the 
conditions  which  forbid  the  growth  of  forests.  Prairies  are 
at  least  of  two  kinds.  Some  are  edaphic — that  is,  they  are 
due  to  local  soil  conditions.  Such  prairies  are  character- 
istic of  the  eastern  prairie  region,  and  even  appear  in  scat- 
tered patches  throughout  the  forest  region  as  far  east  as 
Ohio,  Kentucky,  etc.  They  are  probably  best  explained  as 
representing  old  swamp  areas,  which  at  a  still  more  ancient 
time  were  ponds  or  lakes.  All  the  prairies  of  the  Chicago 
area  are  evidently  edaphic,  being  associated  with  former 
extensions  of  Lake  Michigan.  Other  prairies  are  climatic — 
that  is,  they  are  due  to  general  climatic  conditions.  Such 
prairies  are  characteristic  of  the  western  prairie  region, 
merging  into  the  plains,  and  are  more  puzzling  than  the 
edaphic    prairies.     Among   the   several    explanations   sug- 


■■ 


■*'  :-;* 


-*§ 


3     • 


MESOPIIYTE   SOCIETIES.  241 

gested  perhaps  that  which  refers  the  western  prairies  to 
the  prevailing  dry  winds  is  the  most  prominent. 

The  extensive  plains  of  the  West  develop  the  strong 
and  dry  winds  which  prevail  over  this  prairie  region,  and 
this  brings  about  extremes  of  heat  and  drouth,  in  spite  of 
the  character  of  the  soil.  In  such  conditions  a  tree  in  a 
germinating  condition  could  not  establish  itself.  If  it  is 
protected  through  this  tender  period  it  can  maintain  itself 
afterward,  but  the  drying  winds  forbid  any  plant  with  a 
prolonged  and  sensitive  juvenile  period.  These  prairies, 
therefore,  would  represent  a  sort  of  broad  beach  between 
the  western  plains  and  the  eastern  prairies  and  forests. 

What  seems  to  be  a  confirmation  of  this  view  may  be 
observed  in  certain  north  and  south  valleys  in  the  Missouri 
region  which  lies  on  the  border  between  plains  and  prairies. 
The  eastern  slopes  of  such  valleys,  exposed  to  the  wind 
from  the  plains,  are  without  trees ;  while  on  the  western 
slopes,  protected  from  this  wind,  trees  occur. 

Probably  the  oldest  explanation  of  such  prairies  is  the 
occurrence  of  prairie  fires,  but  this  would  appear  to  be  too 
local  a  cause  for  what  seems  to  be  a  continental  feature. 
Eecently,  however,  the  fire  theory  has  been  revived,  and 
evidence  has  been  brought  forward  to  show  that  in  some 
places,  at  least,  a  forest  growth  would  appear  if  fire  and 
stock  were  kept  out.  In  fact,  the  claim  is  made  that  Ne- 
braska is  becoming  gradually  forest-clad. 

170.  Pastures. — This  term  is  applied  to  areas  drier  than 
natural  meadows,  and  includes  the  meadows  formed  or  con- 
trolled by  man  (see  Fig.  207).  They  may  be  natural,  or 
derived  from  natural  meadow  areas,  or  from  forest  clear- 
ings ;  therefore  they  are  often  maintained  in  conditions 
which,  if  not  interfered  with,  would  not  produce  a  meadow. 
In  general,  the  pasture  differs  from  the  natural  meadow  in 
being  drier,  a  fact  often  due  to  drainage,  and  in  develop- 
ing lower  and  more  open  vegetation.  Naturally  the  plant 
forms  are  prevailingly  grasses,  and  their  cultivation  is  the 


24:2 


PLANT   RELATIONS. 


purpose  of  the  artificial  pasture,  but  the  meadow  tendency 
is  shown  by  the  coming  in  of  perennial  weeds.  The  inva- 
sion of  pastures  by  weeds  suggests  many  interesting  ques- 
tions.     Are  the  weeds   natives  or  foreigners?     Are   they 


Fig.  x>07.  A  juniper  heath  interspersed  with  pastures.  The  growths  of  juniper  are 
very  dense,  excluding  all  other  vegetation,  and  the  grass  or  pasture  areas  are  too 
dry  to  form  real  meadows.— After  Cowi.es. 


annuals  or  perennials  ?  What  is  the  relative  success  of  the 
different  invaders,  and  why  are  some  more  successful  than 
others  ?  A  study  of  pastures  will  also  reveal  the  fact  that 
there  is  great  difference  in  the  vegetation  of  mowed  and 
grazed  pastures.  The  same  effects  are  noted  when  natural 
meadows  are  used  for  grazing. 

B.    Woody  societies. 

These  societies  include  the  various  shrub  and  tree  asso- 
ciations of  mesophyte  areas,  associations  entirely  distinct 
from  the  grass  and  herb  societies. 


MESOPHYTE   SOCIETIES.  2-±3 

171.  Thickets. — The  mesophyte  thickets  are  not  so 
abundant  or  impenetrable  as  the  xerophyte  thickets. 
They  seem  to  be  developed  usually  as  forerunners  of  forest 
vegetation.  An  illustration  of  this  fact  may  be  obtained 
by  noting  the  succession  of  plants  which  appear  on  a 
cleared  area.  After  such  an  area  has  been  cleared  of  its 
trees,  by  cutting  or  by  fire,  it  is  overrun  by  herbs  which 
develop  rapidly  from  the  seed.  Sometimes  these  herbs  are 
tall  and  with  showy  flowers,  as  the  so-called  fire-weed  or 
great  willow  herb.  Following  the  herb  societies  there  is  a 
gradual  invasion  of  coarser  herbs  and  shrubby  plants, 
forming  thickets,  and  finally  a  forest  growth  may  appear 
again. 

In  arctic  and  alpine  mesophyte  regions  the  willow  is 
the  great  thicket  plant,  often  covering  large  areas,  but  in 
temperate  regions  willow  thickets  are  confined  to  stream 
banks  and  boggy  places,  being  the  characteristic  hydro- 
phyte thicket  form. 

The  upland  and  flood-plain  mesophyte  thickets  of  tem- 
perate regions  are  different  in  character.  For  example, 
the  upland  thicket  of  the  Northern  States  very  commonly 
contains  hazel,  birch,  and  aspen  as  dominant  plants ;  while 
the  flood-plain  thicket  is  apt  to  contain,  in  addition  to 
these,  prominent  growths  of  haws  and  wild  crab-apples. 
In  this  same  region  pure  thickets  frequently  occur — that 
is,  thickets  in  which  a  single  form  is  the  prevailing  type, 
as  pure  hazel  thickets  on  uplands,  or  pure  haw  thickets  on 
flood-plains. 

In  the  Southern  States  the  plants  enumerated  above 
may  not  be  the  characteristic  mesophyte  thicket  plants. 
For  example,  in  Kentucky  and  Tennessee  the  dominant 
thicket  plants  are  persimmon,  locust,  redbud,  and 
sassafras. 

172.  Forests  of  temperate  regions. —Deciduous  forests 
are  especially  characteristic  of  temperate  regions.  The 
deciduous  habit,  that  is,  the  habit  of  shedding  leaves  at  a 

17 


244 


PLANT   RELATIONS. 


certain  period,  is  an  adaptation  to  climate.  In  the  tem- 
perate regions  the  adaptation  is  in  response  to  the  winter 
cold,  when  a  vast  reduction  of  delicate  exposed  surface  is 
necessary.  Instead  of  protecting  delicate  leaf  structures 
from  the  severe  cold  of  winter,  these  plants  have  formed 
the  habit  of  dropping  them  and  putting  out  new  leaves 
when  the  favorable  season  returns. 

It  is  instructive  to  notice  how  differently  the  conifers 
(pines,  etc.)  and  the  deciduous  trees  (oaks,  maples,  etc.)  have 

answered  the  problem  of  adaptation 
to  the  cold  of  winter.  The  conifers 
have  protected  their  leaves,  giving 
them  a  small  surface  and  heavy 
walls.  In  this  way  protection  has 
been  secured  at  the  expense  of 
working  power  during  the  season 
of  work.  Reduced  surface  and 
thick  walls  are  both  obstacles  to 
leaf  work.  On  the  other  hand, 
the  deciduous  trees  have  devel- 
oped the  working  power  of  their 
leaves  to  the  greatest  extent,  giving 
them  large  surface  exposure  and 
comparatively  delicate  walls.  It 
is  out  of  the  question  to  protect 
such  an  amount  of  surface  during 
the  winter,  and  hence  the  decidu- 
ous habit.  The  conifers  are  saved 
the  annual  renewal  of  leaves,  but 
lose  in  working  power ;  the  de- 
ciduous trees  must  renew  their  leaves  annually,  but  gain 
greatly  in  working  power. 

It  should  be  remarked  that  leaves  do  not  fall  because 
they  are  broken  off,  but  that  in  a  certain  sense  it  is  a 
process  of  growing  off.  Often  at  the  base  of  the  leaves, 
where   the   separation   is   to   occur,  a   cleavage   region   is 


Fig.  208.  A  section  through  the 
base  of  a  leaf  of  horse-chestnut 
preparing  to  fall  off  at  the  end 
of  the  growing  season.  A 
cleavage  plate  (s)  has  devel- 
oped between  the  woody  bun- 
dle (6)  and  the  surface.  Pres- 
ently this  reaches  the  surface, 
and  only  the  woody  strand 
fastens  the  leaf  to  the  stem. 


MESOPHYTE   SOCIETIES.  245 

gradually  developed  until  the  leaf  is  entirely  separated 
from  the  stem  except  by  a  woody  strand  or  two,  which  is 
easily  broken  (see  Fig.  208).  In  this  way  the  scar  which 
remains  has  really  been  formed  before  the  leaf  falls. 

In  this  process  of  sloughing  off  leaves,  the  plant  cannot 
afford  to  lose  the  living  substance  present  in  the  working 
leaves.  This  substance,  during  the  preparation  for  the 
fall,  has  been  gradually  withdrawn  into  the  permanent 
parts  of  the  plant. 

It  will  be  noticed  that  in  general  deciduous  leaves  are 
thin,  exceedingly  variable  in  form,  and  in  a  general  hori- 
zontal position,  nor  do  they  have  the  firm,  leathery  texture 
of  the  xerophyte  leaves.  All  this  indicates  great  leaf  ac- 
tivity, for,  the  necessity  of  protection  being  removed,  the 
leaf  is  not  impeded  in  its  work  by  the  development  of  pro- 
tective structures. 

One  of  the  most  prominent  features  associated  with  the 
deciduous  habit  is  the  autumnal  coloration.  The  vivid 
colors  which  appear  in  the  leaves  of  many  trees,  just  before 
the  time  of  falling,  is  a  phenomenon  which  has  attracted  a 
great  deal  of  attention,  but  although  it  is  so  prominent,  the 
causes  for  it  are  very  obscure.  It  will  be  noticed  that  this 
autumnal  coloration  consists  in  the  development  of  various 
shades  of  two  typical  colors,  yellow  and  red.  These  colors 
are  often  associated  together  in  the  same  leaf,  and  some- 
times a  leaf  may  show  a  pure  color. 

The  two  colors  hold  a  very  different  relation  in  the  leaf 
cell.  It  is  known  that  the  yellow  is  due  to  the  breaking 
down  of  chlorophyll,  so  that  the  chloroplasts,  which  are 
green  when  active,  become  yellow  when  disorganizing,  and 
finally  bleach  out  entirely.  That  yellow  may  indicate  a 
post  mortem  change  of  chlorophyll  may  be  noticed  in  con- 
nection with  the  blanching  of  celery,  in  which  the  leaves 
and  upper  part  of  the  stem  may  be  green,  the  green  may 
shade  gradually  into  yellow,  and  finally  into  the  pure  white 
of  complete  blanching. 


246  PLANT    RELATIONS. 

The  red  shades,  however,  do  not  seem  to  hold  any  such 
relation  to  the  disorganization  of  chlorophyll.  The  red 
coloring  matter  appears  as  a  stain  in  the  cell  sap,  so  that 
what  might  he  called  the  atmosphere  of  the  active  cell  is 
suffused  with  red.  Certain  experiments  upon  plant  colors 
have  indicated  that  the  presence  of  the  red  color  slightly 
increases  the  temperature  by  absorbing  more  heat.  This 
has  suggested  that  the  red  color  may  be  a  slight  protec- 
tion to  the  living  substance,  which  has  ceased  working 
and  which  is  in  danger  of  exposure  to  cold.  If  this  be 
true,  it  may  be  that  the  same  explanation  will  cover  the 
case  of  the  red  flush  so  conspicuous  in  buds  and  young 
leaves  in  the  early  spring.  It  must  not  be  supposed  that 
the  need  of  protection  has  developed  the  color,  but  that 
since  it  is  developed  it  may  be  of  some  such  service  to  the 
plant.  The  whole  subject,  however,  is  too  indefinite  and 
obscure  to  be  presented  in  any  other  form  than  as  a  bare 
suggestion. 

Even  the  conditions  which  determine  autumnal  colora- 
tion have  not  been  made  out  certainly.  To  many  the  au- 
tumnal coloration  is  associated  with  the  coming  of  frost, 
which  simply  means  a  reduction  of  temperature  ;  others 
associate  it  with  diminishing  water  supply;  still  others 
associate  it  with  the  change  in  the  direction  of  the  rays  of 
light,  which  are  mora  oblique  in  autumn  than  during  the 
active  growing  season.  It  is  certainly  true  that  the  colors 
are  far  more  brilliant  in  certain  years  than  in  others,  and 
that  the  coloration  must  be  connected  in  some  way  with 
the  food  relations  of  the  plants.  Eecent  experiments  have 
shown  that  the  red  coloration  is  largely  dependent  upon  low 
temperature,  which  affects  certain  of  the  food-stuffs,  and 
the  red  stain  is  one  of  the  products. 

The  autumnal  colors  are  notably  striking  in  American 
forests  on  account  of  the  fact  that  in  these  forests  there  is 
the  greatest  display  of  species,  and  hence  not  only  are  more 
colors  produced,  but  they  are  usually  strikingly  associated. 


MESOPHYTE   SOCIETIES.  247 

Not  only  is  protection  during  the  cold  period  secured 
by  deciduous  forests  through  the  falling  of  leaves,  but  the 
development  of  scaly  buds  is  an  adaptation  to  the  same  end 
By  means  of  these  overlapping,  often  hairy,  and  even  var- 
mshed  structures,  delicate  growing  tips  are  protected  dur- 
ing the  cold  season.  The  development  of  cork,  also,  on  the 
older  parts,  is  a  measure  of  protection. 

Although  the  trees  are  the  dominant  plants  of  a  forest 
society,  it  must  not  be  forgotten  that  numerous  other  form 
are  associated  with  them.  At  a  lower  level  stand  the  shrubs 
below  these  the  tall  herbs,  then  the  low  herbs  and  grasses' 
and  finally  close  to  the  soil  mosses  and  lichens". 
These  different  strata,  as  they  are  called,  represent  differ- 
up  oft  d m  i  rnce  to  light' the  lower  strata  bei»g  ™ae 

up  oUhade  plants  as  compared  with  the  upper  strata.     In 

olant   06f  t    ,e  haWt  ^  b6COme  S°  estoblished  *  ™*J 
pUnts  of  the  lower  strata  that  they  depend  upon  the  pros 

outthem      °Vershad0W1"S  str^>  ^d  could  not  live  with- 

The  vernal  habit  is  also  an  interesting  feature  of  decidu- 
ous forests.     It  is  a  matter  of  common  observation  that  the 

Sfns  bl7  °t  "  T™^  fl°WerS  "  °CC"'-S  b  f °™ t.  and  wooded 
glens  before  the  trees  come  into  full  foliage.     The  working 

season  of  these  vernal  plants  is  before  the  dense  foliage  of 
the  forest  shuts  off  the  light.  Accordingly,  they  are  mostly 
geophilous  m  habit  (see  §146),  sending  up  their  shoots  or 
leaves  with  great  rapidity  from  underground  tubers,  root- 
s tocks,  etc.  and  completing  their  vegetative  work  in  the 
short  period  during  which  the  light  is  available.  After  the 
forest  leaves  arc  fully  developed  the  spring  flowers  disap- 
pear waiting  in  their  subterranean  retreats  for  the  next 
short  period  of  activity.  Two  prominent  forms  of  the  ver- 
nal habit  may  be  observed.  The  loaves  may  appear  before 
the  flowers,  as  ,n  Brythronium  and  Hydrophyllum  ;  or  they 

may^pearaftertbeflowcrs,aSin//,vw//,,,  ,,,;,,;,„,„/ ,,]. 

One  of  the  wild  leeks  (Allium  tricoccum)  has  developed  a 


248  PLANT   RELATIONS. 

very  interesting  modification.  It  sends  up  its  rosette  of 
large  and  very  active  leaves  during  the  vernal  season,  and 
when  these  have  disappeared  the  flowers  are  developed  in 
the  forest  shade.  The  significance  of  this  is  that  while 
the  leaves  must  have  the  light  for  their  work,  the  flowers 
can  develop  jnst  as  well  in  the  shade. 

As  in  the  case  of  thickets,  deciduous  forests  may  be 
pure  or  mixed.  A  very  common  type  of  pure  forest  is  the 
beech  forest,  which  is  a  characteristic  dark  forest.  The 
wide-spreading  branches  of  neighboring  beeches  overlap 
each  other,  so  as  to  form  dense  shade.  As  a  consequence, 
in  a  pure  beech  forest  there  is  little  or  no  undergrowth  ;  in 
fact,  no  lower  strata  of  vegetation  until  the  lowest  ones  are 
reached,  made  up  of  grasses  and  mosses.  Another  type  of 
pure  forest,  which  belongs  to  the  drier  regions,  is  the  oak 
forest,  which  forms  a  sharp  contrast  to  the  beech,  in  that 
it  is  a  light  forest,  permitting  access  of  light  for  lower 
strata  of  plants.  Hence  in  such  a  forest  there  is  usually  more 
or  less  undergrowth,  consisting  of  shrubs,  etc.,  which  may 
develop  regular  thickets.  The  typical  American  deciduous 
forest,  however,  is  the  great  mixed  forest,  made  up  of  many 
varieties  of  trees,  such  as  beech,  oak,  elm,  walnut,  hickory, 
gum,  maple,  etc. 

The  deciduous  forests  may  be  roughly  grouped  as  up- 
land and  flood-plain  forests,  the  former  being  less  luxuriant 
and  containing  fewer  types,  the  latter  being  the  highest  ex- 
pression of  forest  development  in  its  region.  A  few  general 
illustrations  may  be  given  as  follows  : 

In  northern  Illinois  the  upland  forest  is  mostly  made 
up  of  three  forms,  white  and  red  oaks  and  shellbark  hick- 
ory ;  while  the  flood-plain  forest  contains  twenty  to  twenty- 
five  tree  forms,  prominent  among  which  are  the  elms  (white 
and  slippery),  linden  (basswood),  cottonwood,  ash,  silver 
maple,  box  elder,  walnut,  and  willows  (see  Fig.  211). 

Farther  south,  from  central  Illinois,  Indiana,  and  Ohio 
southward,  as  well  as  in  the  Alleghanies,  the  flood-plain  for- 


250  PLANT  RELATIONS. 

ests  are  the  richest  known,  containing,  in  addition  to  the 
forms  enumerated  above,  such  prominent  trees  as  the  syca- 
more, beech,  hackberry,  honey  locust,  coffee  tree,  sugar 
maple,  tulip  tree,  buckeye,  etc. 

In  Michigan  and  Wisconsin  the  upland  forests  consist 
prominently  of  beech,  sugar  maple,  and  hemlock,  a  charac- 
fceristic  mixture  of  deciduous  and  evergreen  trees;  while 
the  flood-plain  forests  are  scarcely  at  all  developed. 

In  the  Alleghany  region  and  Xew  England  the  upland 
forests  are  very  extensive  and  complicated,  grading  from 
the  rich  flood-plain  forests  of  the  lower  levels  on  the  one 
hand,  to  the  strictly  xerophytic  forests  (pines  and  black 
oaks)  of  the  higher  levels  on  the  other  hand,  and  dominated 
by  various  oaks  (especially  white,  red,  and  chestnut  oaks), 
chestnuts,  and  hickories  (see  Figs.  209,  210). 

The  flood-plain  forests  of  Xew  England  are  not  so  rich 
as  those  of  the  Alleghany  region  and  Central  West,  the 
dominant  forms  being  elms,  linden,  ash,  maples,  sycamore, 
tulip  tree,  etc. 

173.  Tropical  forests. — The  tropical  forests  may  be 
grouped  under  two  general  heads  :  (1)  the  evergreen  forests, 
and  (2)  the  deciduous  or  monsoon  forests.  The  former  are 
characterized  by  continuous  moisture,  and  are  most  largely 
developed  in  the  East  Indies  and  along  the  Amazon  and  its 
tributaries  in  South  America.  The  deciduous  tropical  for- 
ests are  characterized  by  having  a  period  of  relative  dry- 
ness, during  which  the  leaves  are  shed,  and  usually  border 
the  evergreen  forests. 

A.  Evergreen  forests. — These  rainy  forests  of  the  tropics 
may  be  regarded,  as  Warming  says,  "  as  the  climax  of  the 
world's  vegetation,"  for  the  conditions  in  which  they  are  de- 
veloped favor  constant  plant  activity  at  the  highest  possible 
pressure.  Such  great  forest  growths  are  found  within  the 
region  of  the  trade  winds,  where  there  is  heavy  rainfall, 
great  heat,  and  rich  black  soil.  So  abundant  is  the  precipi- 
tation that  the  air  is  often  saturated  and  the  plants  drip 


252  PLANT   KELATIONS. 

with  moisture.  In  such  conditions  pure  forests  may  oc- 
cur, characterized  by  such  tree  forms  as  the  tree  ferns, 
palms,  or  bamboos.  Only  the  great  mixed  tropical  forest 
will  be  considered.  The  main  characteristics  are  as  fol- 
lows : 

(1)  Absence  of  simultaneous  periodicity. — Perhaps  the 
most  striking  feature,  in  contrast  with  the  deciduous  for- 
ests, is  that  there  is  no  regular  period  for  the  develop- 
ment or  fall  of  leaves.  Leaf  activity  is  possible  through- 
out the  year,  and  there  is  no  time  of  bare  forest,  or  of 
forests  just  putting  out  leaves.  This  does  not  mean  that 
the  leaves  persist  indefinitely,  but  that  there  is  no  regular 
time  for  their  fall  and  formation.  Leaves  are  continually 
being  shed  and  formed,  but  the  trees  always  appear  in  full 
foliage. 

(2)  Density  of  growth. — Such  an  area  is  remarkably 
filled  with  vegetation  stratum,  after  stratum  occurring, 
resulting  in  gigantic  jungles.  The  higher  strata  may  be 
made  up  of  trees  of  different  heights,  below  them  are  shrubs 
of  varying  heights,  then  tall  and  low  herbs,  and  finally 
mosses  and  liverworts.  Among  these  close-set  standing 
forms,  great  vines  or  lianas  climb  and  bind  the  standing 
vegetation  into  an  inextricable  tangle  (see  Figs.  55,  212). 
In  addition  to  these,  hosts  of  aerial  plants  find  lodging 
places  upon  the  tree-trunks  and  vines  (see  Fig.  213).  These 
rainy  forests  of  the  tropics  furnish  the  very  best  conditions 
for  the  development  of  the  numerous  epiphytic  orchids, 
bromelias,  etc.  In  such  conditions  also  numerous  sapro- 
phytes occur.  Such  an  assemblage  of  vegetation  is  to  be 
found  nowhere  else. 

(3)  Number  of  species. — Xot  only  is  there  an  immense 
number  of  individuals,  but  an  extraordinary  number  of 
species  occur.  A  list  of  plants  growing  in  these  forests 
would  show  a  remarkable  representation  of  the  plant 
kingdom. 

(4)  Forms  of  trees. — The   dense   vegetation   results   in 


MESOPHYTE   SOCIETIES. 


255 


straight  leafless  tree-trunks,  so  that  the  leaves  of  trees  are 
mainly  clustered  at  the  tops  of  high  brandies.  The  shade 
is  so  dense  and  the  interference  is  so  great  that  the  devel- 
opment of  low  branches  is  impossible.     It  is  common,  also 


Fig.  213  A  -roup  of  aerial  plants  (epiphytes)  from  a  tropical  forest,  Note  the  vari- 
ous hahits  of  the  epiphytes  attached  to  the  tree-trunks,  and  the  dangling  roots  - 
After  Schimper. 

for  the  larger  trees  to  develop  a  system  of  buttresses  near 
the  base,  and  also  frequently  to  send  out  prop  roots  (see 
Figs.  100,  101). 

(5)  Absence  of  hud  scales.— In  the  deciduous  forest  bud 
scales  are  necessary  to  protect  the  tender  growing  tips  dur- 
ing the  period  of  cold.     The  same  device  would  be  suffi- 


256 


PLANT   RELATIONS. 


cient  to  protect  against  a  period  of  drouth.  In  the  tropical 
forest  there  is  danger  neither  from  cold  nor  drouth,  and  in 
such  conditions  bud  scales  are  not  developed,  and  the  buds 
remain  naked  and  unprotected. 

(G)  Devices  against  too  abundant  rain. — The  abundance 
of  rain  is  in  danger  of  checking  transpiration,  and  as  this 

process  is  essential  to  plant 
activity,  there  are  often 
found  devices  to  prevent 
the  leaves  from  becoming 
saturated.  Many  leaves 
have  cuticles  so  smooth 
and  glazed  that  the  water 
glances  off  without  soaking 
in ;  in  other  cases  a  velvety 
covering  of  hairs  answers 
the  same  purpose;  in  still 
other  cases  leaves  are  gut- 
ter-pointed, that  is,  the  tip 
is  prolonged  as  a  sort  of 
gutter,  and  the  veins  are 
depressed,  the  whole  sur- 
face of  the  leaf  resembling 
a  drainage  system,  so  that 
the  rain  is  conducted  rap- 
idly from  the  surface  (see 
Fig.  214).  These  are  only 
a  few  illustrations  of  many 
devices  against  dangerous 
wetting. 

B.  Deciduous  or  mon- 
soon forests. — In  these  for- 
ests the  same  general  habits  prevail  as  in  the  rainy  evergreen 
forests,  but  to  a  less  degree.  For  example,  the  epiphytes 
and  lianas  are  present,  but  they  are  not  so  numerous  or 
conspicuous.     The     striking     difference,  however,  is    the 


Fig.  214.      A  gutter-pointed  leaf  from    a 
tropical  plant.— After  Schimper. 


MESOPIIYTE   SOCIETIES.  257 

deciduous  habit,  developed  apparently  by  the  regular 
recurrence  of  a  relatively  dry  period,  although  it  may  be 
very  short.  Such  forests  are  usually  adjacent  to  the  ever- 
green forests,  much  as  upland  forests  are  adjacent  to  flood- 
plain  forests. 


INDEX. 


[The  italicized  numbers  indicate  that  the  subject  is  illustrated  on  the  page  cited. 
In  such  case  the  subject  may  be  referred  to  only  in  the  illustration,  or  it  may  be 
referred  to  also  in  the  text.] 


Acacia,  199. 

Achillea,  202. 

Adaptation,  147. 

Adiantum,  27. 

Aeration,  92,  93,  95,  183. 

Agave,  45,  47. 

Agrimony,  121. 

Ailanthus,  116. 

Air,  95,  98,  114,  122,  138. 

Air  cavities,  171.  172,  173,  175. 

Air  passages,  92,  93,  94,  95. 

Air  plants,  97,  98,  99,  100,  101,  246. 

Air  roots,  97,  98,  99,  100. 

Alchemilla,  79. 

Algae,  1,  2,  87,  99,  107,  109,  110, 

111.  113,  150,  171.  172,  177. 
Alkaline  deserts,  255. 
Alpine  plants,  148,  234,  236. 
Amicia,  0. 
Ampelopsis,  63. 
Anemophilons,  122. 
Animals.  119,    121,   122,   123,   145, 

205. 
Annual  habit,  195. 
Annual  rings,  s.'h 
Anthurium,  97. 
Apple,  79. 
Araucaria.  74. 
18 


Arbor  vitae,  139. 
Arctic  plants,  148,  235. 
Arrow-leaf,  186. 
Artillery  plant,  120. 
Ash,  116. 
Aspidium,  55. 
Assimilation,  154,  156. 
Autumn  coloration,  245. 

B 

Bacteria,  189. 
Banana,  88. 
Banyan,  105,  106. 
Barberry,  207. 
Bark,  84. 

Basswood,  116,  201. 
Beach,  209,  210. 
Beach  marshes,  19,  26. 
Beach  pea,  US. 
Beach  societies,  209. 
Bean,  140. 
Bearberry,  212. 
Beech  drops,  157. 
Beech  forest,  145. 
Beggar  ticks,  119,  121. 
Begonia.  25,  208. 
Bellflower,  10.  so. 
Bidens,  119. 
Bignonia,  115. 


259 


260 


INDEX. 


Bilbergia,  136. 

Birches.  71. 

Black  moss,  96,  101. 

Bladder  wort,  173. 

Blade,  35. 

Bloodroot,  195. 

Bogs,  143. 

Box  elder,  S4. 

Bramble,  94. 

Branched  leaves,  19,  20,  21,  23. 

Buds,  70,  73,  75,  141,  247. 

Bulbs,  73,  75,  81.     * 

Bulrush,  142,  148,  185,  186,  207. 

Burdock,  121,  122. 

Bush,  227. 

Bush  clover,  43. 

Buttercup,  185. 

Buttresses,  103,  104. 


C 


Cactus  deserts,  217,  222, 

Cactus  forms,  45,  4<>,  47,  146,  202, 

207,  215,  216,  217,  218,  219,  222. 
Calyx,  78,  79,  SO,  125. 
Campanula,  19,  SO. 
Caoutchouc,  136. 
Carbohydrates,  153,  156. 
Carbon,  153. 

Carbon  dioxide,  30,  151,  153. 
Cardon  forests,  231. 
Carnation,  ,;.'. 
Carnivorous  plants,  155,  156,  157. 

173,  189. 
Carpel.  78,  79,  80,  125. 
Carrot,  120. 
Castor-oil  bean,  73. 
Casuarina,  231. 
Catalpa,  117. 
Catchfly,  136. 

Cat-tail  flag,  142,  148,  185,  186. 
Cercis,  10, 


Change  in  temperature,  145. 
Chaparral,  227. 
Chlorophyll,  6,  8,  149,  152. 
Chloroplasts,  39,  107,  152,  205,  208, 

209,  245. 
Chrysanthemum,  23. 
Cilia,  109,  111. 
Claytonia,  196. 
Cleistogamous,  130. 
Clematis,  113. 
Climbing  stems,  60,  61,  62,  63,  64, 

102. 
Clinging  roots,  99,  102. 
Clinia,  209. 
Cocklebur,  120,  121. 
Compass  plants,  10,  12,  197,  198. 
Compound  leaves,  19,  20,  21,  23. 
Conducting  tissue,  171. 
Conifer  forests,  226,  227,  228,  229, 

230. 
Conifers,  83,  190,  191,  225,  226. 
Cork,  247. 
Corn,  85,  90. 
Corolla,  78,  79,  SO. 
Cortex,  83,  84,  93,  94,  107,  108. 
Cottonwood,  70. 
Cotyledons,  50,  51,  73,  139,  140. 
Crevice  plants,  94,  209. 
Cuticle,  42,  205. 
Cycad,  22. 
Cycloloma,  117. 
Cypress  knees,  95,  96,  183. 
Cypripedium,    132,    133,  13Jh   135, 

136. 
Cytisus,  206. 


Dandelion,  82,  U4,  117. 
Darlingtonia,  157. 
Date  palm,  86. 
Dead-nettle,  SO. 


INDEX. 


261 


Deciduous  forests,  243. 

Deciduous  habit,  143,  196,  243,244- 

Deserts,  221,  222,  223. 

Desiccation,  11)4. 

Desmodium  gyrans,  49. 

Destruction  of  plants,  148. 

Diatoms,  174. 

Dicotyledons,  85,  83,  116. 

Differentiation,  3. 

Digestion,  154,  156. 

Dionaea,  160,  161. 

Dodder,  106,  107,  157. 

Dog-tooth  violet,  144- 

Dragon  tree,  15. 

Drainage,  143,  145. 

Drosera,  158,  159. 

Drouth,  193. 

Duckweed,  97,  175. 

Dunes,  145,  201,  209,  211,  212. 

Dwarf  growths,  203. 


E 


Easter  lily,  14. 

Eeheveria,  17. 

Ecological  factors,  163. 

Ecology,  4,  149. 

Eel  grass,  IS4. 

Egg,  110,  111. 

Elaters,  118. 

Elatine,  93. 

Elm,  63,  67,  68,  75. 

Embryo,  111,  139. 

Entomophilous,  122,  12:5. 

Epidermis,  37.  Jfi,  .',1.  4-\  83,  84, 

107,  170,  205.  208,  209. 
Epilobium,  112,  118,  128,  1:55. 
Epiphyte,  209. 
Equisetum,  111,  203. 
Erect  stems,  62,  65,  66,  67,  6S,  69, 

70,  71. 
Erica,  200. 


Erythronium,  144. 
Euphorbia,  20%. 


V 


Ferns,  55,  56,  85,  SS,  100,  111,  113, 

119. 
Fertilizing,  145. 
Ficus,  8. 

Figwort,  128,  135. 
Fireweed,  112,  113,  128,  135,  243. 
Fittonia,  37,  152. 
Fixed  light  position,  197. 
Flag,  126,  133. 
Floating  stems,  59. 
Floats,  171,  172,  173. 
Flowers,  76,  78,  140. 
Foliage  forests,  227. 
Foliage  leaves,  6,  28,  139. 
Forest  clearing,  143,  145. 
Forests,  190,  226,  227,  228,  229. 
Fruit,  113,  114,  115,  116,  117,  US, 

119,  120.  121,  122. 
Fucus,  171. 
Functions,  3. 
Fungi,  87,  107,  109,  110. 
Furze,  205. 


G 


Galium,  121. 

Gamete,  110,  111,  112,  113. 

Geophilous  habit,  55,  56,  7:5.  74.  75, 

76,  77,  78,  81,  195,  196,  2:57. 
Geotropism,  69,  91,  138. 
Germination,  111,  138,  139,  140. 
Gorse,  205. 
Grape  vine,  61. 
Grass,  187,  197,  216,  236. 
Gravity,  91. 
Guard  cells,  .As'. 
Gymnosperms,  115. 


262 


INDEX. 


II 


Ilabenaria,  127. 

Hairs,  43,  92,  136,  14G,  202,  203. 

Halophytes,  109.  24!). 

Harebell,  19,  SO. 

Hawthorn,  36. 

Heart-wood,  151. 

Beat,  112,  138,  145,  164. 

Heath  plants,  189,  200,  214. 

Helianthemum,  236. 

Heliotropism,   12,  13,   68,   72,  73, 

139. 
Hemlock,  190. 
Horse-chestnut,  244. 
Hosts,  106. 
House  leek,  19. 
Houstonia,  129,  135. 
Huckleberry,  214. 
Hudsonia,  212. 
Hura  crepitans,  120. 
Hydrogen,  153. 
Hydrophytes,  168,  170,  174. 
Hydrotropism,  91,  138. 


I 


Insects  and  flowers,  123. 
Iris,  126,  133. 
Isoetes,  94,  95,  208. 
Ivy,  99. 


Juncus,  77. 
Juniper,  51,  242. 


Lactuca,  12,  197,  198. 
Lady-slipper,  132, 133,  134, 135i 136- 


Lakes,  143,  148. 

Laminaria,  177. 

Larch,  178,  190. 

Latex,  136. 

Leafless  forests,  227. 

Leaflet,  19. 

Leaf- relation,  5:j. 

Lemna,  97. 

Lespedeza,  43. 

Lianas,  60,  61,  62,  63,  64,  102, 

246. 
Lichens,  194,  209,  214. 
Life- relations,  4,  7,  8,  53,  77. 
Light,  143,  167,  197. 
Light-relation,  7,  8. 
Lily,  38,  40. 
Live-for-ever,  IS. 
Live  oak,  101. 
Liverworts,  118. 
Locomotion,  113. 
Locust,  207. 
Long  moss,  96,  101. 
Loosestrife,  130,  135. 
Lotus,  ISO. 


M 


Mangroves,  192,  192a. 
Maple,  26,  115,  110. 
Mara nt a,  38. 
Marchantia,  107. 
Meadows,  237,  238. 
Mechanical  tissue,  172. 
Mesophyll,  38,  39,  40  41,  42,  152. 
Mesopliytes.  168,233. 
Migration,  58,  75,  147. 
Mildew,  109,  157. 
Milkweed,  117. 
Mimosa.  199. 
Mistletoe,  107. 
Mold,  109. 

Monocotyledons,   35,   85,   88,    116, 
186, 


INDEX. 


188, 


Moors,  187,  188. 

Mosaic  arrangement,  25,  .7.  8j 

Mosses,  87,  107,  110,  113,  118, 

194,  209,  214. 
Motile  leaves,  9,  10,  11,  49,  198, 

199. 
Mould,  109. 
Mullein,  43,  44. 
Mushrooms.  157. 


N 


Nectar,  123,  158. 
Nelumbium,  180. 
Nicotiana,  SO. 
Nightshade,  26. 
Nitrogen,  153. 
Nodes,  54. 
Nuphar,  92. 
Nutrition,  3,  149. 
Nymphaea,  17 S,  ISO. 


Oak,  69,  101. 

Oak  forest,  145,  248. 

(Edogoniuni,  111. 

Orchids,  98,  99,  126,  127,  132,  133, 

l.i',.  135,  136,  189. 
Organs,  3. 
Ornithogalum,  SI. 
Ovary,  79,  so,  125. 
Ovules,  78,  79,  SO. 
Oxalis,  10,  50.  199. 
Oxygen,  29,  138,153. 


Palisade  tissue,  :!!). 
Palms,  86,  87,  230. 
Pandanus,  103. 


;/.  205. 


Parasites,  106,  150. 

Passion  vine,  62. 

Pastures,  241,  242. 

Pellionia,  24. 

Pentsteinon,  137. 

Peony,  7S. 

Petals,  7S,  79,  80. 

Petioles,  15,  26,  35,  55. 

Phlox,  80. 

Photosynthesis,   28,   29,   150,    152, 

153,  156. 
Physiology,  149. 
Pickerel  weed,  181,  182. 
Pines,  63,  65,  66,  112,  115,  117,  190, 

227,  229,  230. 
Pirus,  79. 
Pistil,  77,  79,  SO. 
Pitcher  plant,  155,  156,  157,  158. 
Pith,  83,  84,  107. 
Plains,  213,  215,  216. 
Plankton,  174. 
Plant  body,  2. 
Plant   societies,    1,    146,    162,  168, 

174. 
Plastid,  152. 
Platyceriuni,  100. 
Plumes,  112,  113.  114,  116,  117. 
Plumule,  51,  140. 
Pollen,     77,    111,    112,    115, 

123. 
Pollination,  77,  115,  122,  123. 
Polygonatum,  35. 
Ponds.  142,  175.  778,  ISO,  I84. 
Pondweed,  176,  181.  IS.'. 
Potato.  74.  76. 
Potentilla,  .;./.  79. 
Prairies,  208,  222,  237.  240. 
Prickles,  146. 

Prickly  lettuce.  12.  197,  198. 
Primrose,  137. 
Procumbent  stem.  57. 
Profile  position,  197,  198. 
Pronuba,  130,  131. 


121, 


L>«'4 


IXDEX. 


Prop  roots,  99,  103,  104,  105,  106, 

255. 
Protandry,  128,  135. 
Protection   of   leaves,  9,  10,  11,  12, 

41.  J#,  43,  48,  49. 
Proteids,  153,  156,  189. 
Protogyny,  128,  135. 
Protoplasm,  154,  156. 
Ptelea,  115. 
Pull-balls,  157. 

Q 

Quillwort,  94,  95,  208. 


R 


Rain,  51,  256. 

Ranunculus,  185. 

Raspberry,  91. 

Receptacle,  79,  81,  114. 

Redbud,  10. 

Reed  grass,  142,  185,  186. 

Reed  swamps,  185. 

Reproduction,  3,  109. 

Respiration,  32,  154,  156. 

Rhizoids,  107. 

Rivalry,  146. 

Robinia,  125,  126,  133,  207. 

Rock-rose,  236. 

Rock  societies,  209,  210. 

Roots,  89,  90,  95,  98,  99,  138,  139, 

171. 
Root-cap,  108. 
Root-hairs.  90. 
Rootstalk,  55,  5G,   75,    76,   77,  78, 

195. 
Rose  acacia,  125,  126,  133. 
R«»scttc  habit,  16,  17,  IS,  19,  47,  94, 

15S,  160,  209,  237. 
Rosinweed,  10,  197,  198. 
Rubber  tree,  104. 
Runners,  57,  93. 
Rusts,  157. 


Sage  brush,  216. 

Sagittaria,  186. 

Saintpaulia,  16. 

Salt  deserts,  231. 

Salt  steppes,  231. 

Sand  societies,  209. 

Sandy  fields,  209,  212. 

Sanguinaria,  195. 

Saprophytes,  150,  189. 

Sap-wood,  151. 

Sargassum,  172. 

Sarracenia,  155, 156,  158. 

Saxifrage,  58. 

Scale  leaves,  70,  75. 

Scales,  141. 

Scouring  rush,  203. 

Screw  pine,  103. 

Scrub,  227. 

Seaweeds,  1,  2,  87,  99. 

Sedges,  187. 

Seed-dispersal,  112,  113,  114,  116, 

117,  US,  119,  120. 
Seed-plants,  111,  119,  121. 
Seeds,  111,  112,  113,  115,  138,  139, 

140. 
Selaginella,  26,  100,  194. 
Sempervivum,  19. 
Senecio,  114. 

Sensitive  plants,  11,  48,  50,  199. 
Sepals,  78,  79.  so. 
Shepherdia,  44- 
Shoots,  53. 

Silphium,  10,  197,  198. 
Smilax,  61. 
Snapdragon,  SO,  137. 
Soil.   90,   94,    145,    151,    166,    214, 

224. 
Solomon's  seal,  35,  76. 
Spanish  needle,  119,  121. 
Sphagnum,  188. 


INDEX. 


265 


Sphagnum-bogs,  208. 

Sphagnum-moors,  188. 

Spines,  146,  204. 

Spirogyra,  110. 

Spongy  tissue,  39,  40. 

Spore  case,  55,  118,  119. 

Spore-dispersal,  109,  111,  112,  113, 

114,  118. 
Spores,  109,  110,  111,112. 
Spring  beauty,  196. 
Spring  plants,  143,  144. 
Squash  seedlings,  50. 
Squirting  cucumber,  120. 
Staghorn  fern,  100. 
Stamens.  78,  79,  80,  125. 
Starch.  153. 
Star  cucumber,  61. 
Star-of-Bethlehem,  81. 
Stem,  54,  83,  139. 
Steppes,  216. 
Stigma,  SO,  125. 
Stipules,  35. 
Stomata,  38,  40,  206. 
Strawberry  plant,  57,  58.  93. 
Struggle  for  existence,  142. 
Style,  SO,  125. 
Subterranean  stems,  54,  55,  56,  76, 

77,  78. 
Succulent  plants,  222. 
Sugar,  153. 
Sundew,  158.  159. 
Sunflower.  72. 
Swamp-forest,  190.  191. 
Swamp-moors,  187. 
Swamp-thickets,  188. 
Swamps,  183. 


Tamarack,  178,  190. 
Tap  root,  93. 
Tax  us,  42. 


Teasel,  136. 

Telegraph  plant,  49. 

Temperature,  145. 

Tendrils,  61,  62,  63. 

Thallus,  107. 

Thickets,  188,  224,  243. 

Thistle,  117. 

Thorns,    146,    204,   205,   206,   207, 

224. 
Thuja,  139. 
Tilia,  116,  Jul. 
Tillandsia,  96,  101. 
Toad-flax,  80. 
Toadstools,  149. 
Tobacco,  SO. 
Touch-me-not,  119. 
Tragacanth,  206. 

Transpiration,  31,  33,  154,  193, 256. 
Tropical  forest,  254. 
Trumpet  creeper,  99. 
Tubers,  74,  76,  196. 
Tumbleweeds,  117,  220. 
Turf-building,  185. 


U 

Ulex,  205. 
Ulothrix,  109,  111. 
Utricularia,  173,  174. 


Vallisneria.  IS4. 

Vascular   bundles,  83,  84,  92,  93, 

94.  107,  108,  151.  171. 
Vegetative  multiplication,  109. 
Veins,  35,  36,  37,  40,  151. 
Velamen,  99. 
Venation,  35, 36,  37. 
Victoria,  180. 
Violet,  117,  119. 


266 


INDEX. 


W 


Walnut,  82. 

Water,   90,    92,   94,   95,   113,   138, 

142,  L51,  163,  193,  206,  250. 
Water  lily,  178,  180,  181. 
Water  reservoirs,  200,  208,  209. 
Weeds,  147. 
Willow,  35,  243. 
Wind.  95,  98,  114,  122,  167. 
Wings,  112,  115,  110. 


Witch  hazel,  118,  119. 
Woodbine,  01,  03. 

X 

Xerophytes,  168,  193,  208. 
Xerophyte  structure,  207. 


Yew,  42. 

Yucca,  45,  47,  130, 131,  220. 


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groups,  and  their  relations  to  other  forms  of  life — all  of  which  constitute  the 
economic  and  sociological  phases  of  plant  study. 

Plant  Structures.     A  Second  Book  of  Botany.     12010. 
Cloth,  $1.20. 

This  volume  treats  of  the  structural  and  morphological  features  of  plant 
life  and  plant  growth.  It  is  intended  to  follow  "  Plant  Relations,"  by  the 
same  author,  but  may  precede  this  book,  and  either  may  be  used  independ- 
ently for  a  half-year's  work  in  botanical  study.  "  Plant  Structures"  is  not 
intended  for  a  laboratory  guide,  but  a  book  for  study  in  connection  with 
laboratory  work. 

Plant  Studies.     An  Elementary  Botany.     i2mo.     Cloth, 

$1  25. 

This  book  is  designed  for  those  schools  in  which  there  is  not  a  sufficient 
allotment  of  time  to  permit  the  development  of  plant  Ecology  and  Morphol- 
ogy as  outlined  in  "  Plant  Relations  "  and  "  Plant  Structures,"  and  yet  which 
are  desirous  of  imparting  instruction  from  both  points  of  view. 

Plants.     A  Text-Book  of  Botany.      121110.     Cloth,  $1.80. 

Many  of  the  high  schools  as  well  as  the  smaller  colleges  and  seminaries 
that  devote  one  year  to  botanical  work  prefer  a  single  volume  covering  the 
complete  course  of  study.  For  their  convenience,  therefore,  "  Plant  Rela- 
tions "  and  "  Plant  Structures  "  have  been  bound  together  in  one  book,  under 
the  title  of  "Plants." 

An  Analytical  Key  to  some  of  the  Common  Wild 
and  Cultivated  Species  of  Flowering  Plants. 

i2mo.      Limp  cloth,  60  cents. 

An  analytical  key  and  guide  to  the  common  flora  of  the  Northern  and 
Eastern  States,  as  its  title  indicates.  May  be  used  with  any  text-book  of 
botany. 

D.     APPLETON     AND     COMPANY,     NEW     YORK. 


TWENTIETH  CENTURY  TEXT  BOOKS. 

A  History  of  the  American  Nation. 

By  Andrew  C.  McLaughlin,  Professor  of 
American  History  in  the  University  of  Michi- 
gan. With  many  Maps  and  Illustrations.  i2mo. 
Cloth,  $1.40  net. 

"  One  of  the  most  attractive  and  complete  one- volume  his- 
tories of  America  that  has  yet  appeared." — Boston  Beacon. 

"  Complete  enough  to  find  a  place  in  the  library  as  well  as  in 
the  school." — Denver  Republican. 

"This  excellent  work,  although  intended  for  school  use,  is 
equally  good  for  general  use  at  home." — Boston  Transcript. 

"It  should  find  a  place  in  all  historic  libraries." — Toledo 
Blade. 

"Clearness  is  not  sacrificed  to  brevity,  and  an  adequate 
knowledge  of  political  causes  and  effects  may  be  gained  from  this 
concise  history." — New  York  Christian  Advocate. 

"A  remarkably  good  beginning  for  the  new  Twentieth  Cen- 
tury Series  of  text-books.  .  .  .  The  illustrative  feature,  and 
especially  the  maps,  have  received  the  most  careful  attention, 
and  a  minute  examination  shows  them  to  be  accurate,  truthful, 
and  illustrative." — Philadelphia  Press. 

"  The  work  is  up  to  date,  and  in  accord  with  the  best  modern 
methods.  It  lays  a  foundation  upon  which  a  superstructure  of 
historical  study  of  any  extent  may  be  safely  built." — Pittsburg 
Times. 

"A  book  of  rare  excellence  and  practical  usefulness." — Salt 
Lake  Tribune. 

"The  volume  is  eminently  worthy  of  a  place  in  a  series  des- 
tined for  the  readers  of  the  coming  century.  It  is  highly 
creditable  to  the  author." — Chicago  Evening  Post. 

D.     APPLETON     AND      COMPANY,      NEW      YORK. 


McMASTER'S    FIFTH    VOLUME. 


History  of  the  People   of  the  United 
States. 
By  Prof.  John  Bach  McMaster.    Vols.  I,  II,  III, 
IV,  and  V  now  ready.      8vo.     Cloth,  with   Maps, 
$2.50  per  volume. 

The  fifth  volume  covers  the  time  of  the  administrations  of 
John  Quincy  Adams  and  Andrew  Jackson,  and  describes  the 
development  of  the  democratic  spirit,  the  manifestations  of  new 
interest  in  social  problems,  and  the  various  conditions  and  plans 
presented  between  1821  and  1830.  Many  of  the  subjects  in- 
cluded have  necessitated  years  of  first-hand  investigations,  and 
are  now  treated  adequately  for  the  first  time. 

"John  Bach  McMaster  needs  no  introduction,  but  only  a  greeting.  .  .  . 
The  appearance  of  this  fifth  volume  is  an  event  in  American  literature 
second  to  none  in  importance  this  season." — New  York  Times. 

"This  volume  contains  576  pages,  and  every  page  is  worth  reading 
The  author  has  ransacked  a  thousand  new  sources  of  information,  and  has 
found  a  wealth  of  new  details  throwing  light  upon  all  the  private  and  public 
activities  of  the  American  people  of  three  quarters  of  a  century  ago." — 
Chicago  Tribune. 

"  In  the  fifth  volume  Professor  McMaster  has  kept  up  to  the  high  standard 
he  set  for  himself  in  the  previous  numbers.  It  is  hard  to  realize  thoroughly 
the  amount  of  detailed  work  necessary  to  produce  these  books,  which  con- 
tain the  best  history  of  our  country  that  has  yet  been  published."— Fhi ladel- 
phia  Telegraph. 

"The  first  installment  of  the  history  came  as  a  pleasant  surprise,  and 
the  later  volumes  have  maintained  a  high  standard  in  regard  to  research 
and  style  of  treatment." — Neiv  York  Critic. 

"A  monumental  work.  .  .  .  Professor  McMaster  gives  on  every  page 
ample  evidence  of  exhaustive  research  for  his  facts."— Rochester  Herald. 

"The  reader  can  not  fail  to  be  impressed  by  the  wealth  of  material  out 
of  which  the  author  has  weighed  and  condensed  and  arranged  his  matter." 
— Detroit  Free  Press, 

"Professor  McMaster  is  our  most  popular  historian.  ...  He  never 
wearies,  even  when  dealing  with  subjects  that  would  be  most  wearisome 
under  clumsier  handling.  This  fifth  volume  is  the  most  triumphant  evi- 
dence of  his  art."— New  York  Herald. 

D.     APPLETON     AND      COMPANY,      NEW     YORK. 


APPLETONS'  WORLD  SERIES. 


A  New  Geographical  Library. 

Edited  by  H.  J.  Mackinder,  M.  A.,  Student  of  Christ  Churchy 
Reader  in  Geography  in  the  University  of  Oxford,  Principal  of 
Reading  College.      i2mo.      Cloth,  $1.50  each. 

A  COMPLETE  ACCOUNT  OF  THE  WORLD. 
The  series  will  consist  of  twelve  volumes,  each  being  an  essay  de- 
scriptive of  a  great  natural  region,  its  marked  physical  features,  and 
the  life  of  its  people.     Together,  the  volumes  will  give  a  complete 
account  of  the  world,  more  especially  as  the  field  of  human  activity. 

LIST    OF    THE    SUBJECTS    AND    AUTHORS. 

i.  BRITAIN  AND  THE  NORTH  ATLANTIC.     By  the  Editor. 

2.  SCANDINAVIA  AND  THE  ARCTIC  OCEAN.     By  Sir  Clements 

R.  Markham,  K.  C.  B.,  F.  R.  S.,  President  of  the  Royal  Geographical 
Society. 

3.  THE  ROMANCE  LANDS  AND  BARBARY.    By  Elisee  Reclus, 

author  of  the  "  Nouvelle  Geographie  Universelle." 

4.  CENTRAL  EUROPE.     By  Dr.  Joseph  Partsch,  Professor  of  Geog- 

raphy in  the  University  of  Breslau. 

5.  AFRICA.      By  Dr.   J     Scott   Keltie,  Secretary  of  the  Royal  Geo- 

graphical Society  ;  Editor  of  "  The  Statesman's  Year-Book." 

6.  THE    NEAR  EAST.     By  D.  G.  Hogarth,  M.  A.,  Fellow  of  Magda- 

len College,  Oxford  ;  Director  of  the  Biitish  School  at  Athens  ;  Author 
of  "A  Wandering  Scholar  in  the  Levant." 

7.  THE    RUSSIAN   EMPIRE.     By  Prince  Krapotkin,  author  of  the 

articles  "  Russia"  and  "Siberia"  in  the  Encyclopedia  Britannica. 

8.  THE  FAR  EAST.     By  Archibald  Little. 

9.  INDIA.     By  Sir  T.  Hungerford  Holdich,  K.  C.  I.  E.,  C.  B.,  R.  E., 

Superintendent  of  Indian  Frontier  Surveys. 

co.  AUSTRALASIA  AND  ANTARCTICA.  By  Dr.  H.  O.  Forbes,  Cu- 
rator of  the  Liverpool  Museum  ;  late  Curator  of  the  Christ  Church 
Museum,  N.  Z.  ;  Author  of  "  A  Naturalist's  Wanderings  in  the  East- 
ern Archipelago." 

ii,  NORTH  AMERICA.     By  Prof.  I.  C.  Russell,  University  of  Michigan. 

12.  SOUTH  AMERICA.  By  Prof.  John  C.  Branner,  Vice-President  of 
Leland  Stanford  Junior  University. 

MAPS  by  J.  G.  Bartholomew. 
D.      APPLETON     AND     COMPANY,      NEW     YORK. 


A  WORK  OF  GREAT  VALUE. 

The  International  Geography. 

By   Seventy    Authors,    including    Right  Hon.    James 

Bryce,  Sir  W.  M.  Conway,  Prof.  W.  M.  Davis,  Prof. 

Angelo  Heilprin,  Prof.  Fridtjof  Nansen,  Dr.  J.  Scott 

Keltie,  and   F.    C.    Selous.      With   488  Illustrations. 

Edited   by   Hugh    Robert    Mill,    D.  Sc.  8vo.      1088 
pages.     Cloth,  $3.50. 

"  Can  unhesitatingly  be  given  the  first  place  among  publications  of 
its  kind  in  the  English  language.  ...  An  inspection  of  the  list  of  asso- 
ciate authors  leads  readily  to  the  conclusion  that  no  single  volume  in 
recent  scientific  literature  embodies,  in  original  contributions,  the  labor 
of  so  many  eminent  specialists  as  this  one.  .  .  .  The  book  should  find 
a  place  in  every  library,  public  or  private,  that  contains  an  alias  or 
gazetteer." — The  Nation. 

11  The  attempt  to  present  in  one  volume  an  authoritative  modern 
summary  of  the  whole  of  geography  as  fully  as  space  would  permit  has 
been  admirably  successful." — New  York  Sun. 

"  In  brief,  it  may  be  said  to  be  both  a  reference  book  and  a  con- 
nected geographical  history  of  the  modern  world,  something  that  any 
one  can  read  with  profit  in  addition  to  finding  it  of  constant  value  in 
his  library." — Chicago  Evening  Post. 

"In  his  entirely  studious  moments  the  geographer  cherishes  above 
all  things  facts  and  accuracy.  He  must,  therefore,  value  very  highly 
a  work  like  the  '  International  Geography.'  It  should  be  precious  alike 
to  the  specialist  and  to  the  beginner.  .  .  .  Small  but  adequate  maps  are 
constantly  introduced,  and  there  is,  finally,  a  splendid  index." — New 
York  Tribune. 

"Simply  invaluable  to  students,  teachers,  and  others  in  need  of 
such  a  book  of  reference." — Washington  Times. 

"  Not  only  as  complete  as  the  limits  would  allow,  but  is  strictly 
up  to  date." — San  Francisco  Argonaut. 

D.     APPLETON     AND      COMPANY,      NEW     YORK. 


By  DAVID   STARR   JORDAN,  Ph.  D. 
Animal  Life. 

A  First  Book  of  Zoology.  By  David  Starr  Jordan,  M.  S., 
M.  D.,  Ph.  D.,  LL.  D.,  President  of  Leland  Stanford  Junior 
University,  and  Vernon  L.  Kellogg,  M.  S.,  Professor  of  Ento- 
mology in  Leland  Stanford  Junior  University.  i  2ino.  Cloth, 
Si. 20. 

This  book  gives  an  account  in  an  elementary  form  of  animal 
ecology — that  is,  of  the  relations  of  animals  to  their  surroundings. 
It  treats  of  animals  from  the  standpoint  of  the  observer,  and  at- 
tempts to  show  the  student  why  the  present  conditions  and  habits 
of  animal  life  are  as  we  find  them.  It  explains  how  the  infinite 
variety  of  animal  form  and  mode  of  life  is  the  inevitable  outcome 
of  the  struggle  for  existence  under  changing  conditions  and  envi- 
ronments. Beginning  with  the  amoeba,  the  simplest  form  of  cell 
life,  it  traces  the  evolution  of  animal  variations  and  adaptations 
through  successive  stages  of  development, -until  the  highest  speciali- 
zation and  the  most  complex  organization  are  reached  in  man. 

The  book  is  designed  from  the  outset  to  make  the  student  an 
independent  observer  and  thinker.  It  treats  of  the  phase  of  zool- 
ogy that  appeals  most  strongly  to  the  interest  of  the  young  learner, 
and  in  a  way  to  make  the  study  a  most  pleasant  and  profitable  one. 
It  is  intended  to  provide  work  for  one  half  year  in  the  ordinary 
high-school  course,  and  is  to  be  followed  by  a  second  volume, 
"Animal  Forms,"  treating  of  structure,  to  complete  a  vear's 
study  when  this  period  is  assigned  to  the  subject.  The  topics  as 
treated  are  elastic,  however,  and  either  book  can  be  made  to  cover 
a  somewhat  longer  or  shorter  time,  if  desired. 

The  illustrations,  which  have  been  prepared  expressly  for  the 
work,  are  of  an  especially  attractive  and  instructive  character,  and 
add  conspicuously  to  its  distinctive  features.      Like  the  other  vol 
umes  of  the  Twentieth  Century  Text-Books,  it  is  accompanied  by 
a  brief  manual  containing  hints  to  teachers,  references,  etc. 

D.      APPLETON      AND      COMPANY,      NEW     YORK. 


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